Reland of Mojo: Eliminate duplicate C API symbols (patchset chromium#1 …

…id:1 of https://codereview.chromium.org/2047813003/ )

Reason for revert:
Reverting this didn't fix the failure after all

Original issue's description:
> Revert of Mojo: Eliminate duplicate C API symbols (patchset chromium#3 id:80001 of https://codereview.chromium.org/2044023004/ )
>
> Reason for revert:
> tentative revert due to linker errors. this doesn't make sense. nothing makes sense. RIP sanity
>
> Original issue's description:
> > Mojo: Eliminate duplicate C API symbols
> >
> > This gets rid of the duplicate definitions of public
> > Mojo C API symbols, instead making mojo/public/c/system
> > the singular source of these definitions. This allows
> > targets to be linked against mojo/public libraries
> > without requiring any additional EDK dependencies at
> > build time.
> >
> > To facilitate this the EDK uses the same set of API
> > thunks as the public library, and the public library
> > exposes a special embedder API (distinct from the DSO
> > MojoSetSystemThunks call used by the native app loader)
> > for setting these thunks. The MojoSetSystemThunks API
> > is moved to a module which is only linked directly into
> > app DSOs.
> >
> > Because mojo/public/c/system and mojo/edk/system are
> > now both components, and because there are no longer
> > redundant exports between the two, it's now safe for
> > targets to depend on either one or both of them as
> > needed without encountering duplicate definitions.
> >
> > Also some opportunistic cleanup of Mojo build rules.
> >
> > BUG=612500
> >
> > Committed: https://crrev.com/464e306a82ead839f02f1930a684400228b72495
> > Cr-Commit-Position: refs/heads/master@{#398605}
>
> [email protected]
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=612500
>
> Committed: https://crrev.com/e8e242161954d86ce8e064f8a6515bc818d5b600
> Cr-Commit-Position: refs/heads/master@{#398614}

[email protected]
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=612500

Review-Url: https://codereview.chromium.org/2048053003
Cr-Commit-Position: refs/heads/master@{#398622}

mojo/edk/embedder/BUILD.gn

@@ -37,6 +37,7 @@ source_set("embedder") {
     "embedder.h",
     "embedder_internal.h",
     "entrypoints.cc",
+    "entrypoints.h",
 
     # Test-only code:
     # TODO(vtl): It's a little unfortunate that these end up in the same
@@ -46,12 +47,7 @@ source_set("embedder") {
     "test_embedder.h",
   ]
 
-  defines = [
-    "MOJO_SYSTEM_IMPL_IMPLEMENTATION",
-    "MOJO_SYSTEM_IMPLEMENTATION",
-  ]
-
-  configs += [ "//mojo/edk/system:system_config" ]
+  defines = [ "MOJO_SYSTEM_IMPL_IMPLEMENTATION" ]
 
   public_deps = [
     ":delegates",
@@ -98,8 +94,6 @@ source_set("platform") {
 
   defines = [ "MOJO_SYSTEM_IMPL_IMPLEMENTATION" ]
 
-  configs += [ "//mojo/edk/system:system_config" ]
-
   public_deps = [
     "//mojo/public/cpp/system",
   ]
@@ -131,8 +125,6 @@ source_set("delegates") {
 
   defines = [ "MOJO_SYSTEM_IMPL_IMPLEMENTATION" ]
 
-  configs += [ "//mojo/edk/system:system_config" ]
-
   public_deps = [
     "//mojo/public/cpp/system",
   ]

mojo/edk/embedder/embedder.cc

@@ -15,6 +15,7 @@
 #include "base/task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "mojo/edk/embedder/embedder_internal.h"
+#include "mojo/edk/embedder/entrypoints.h"
 #include "mojo/edk/embedder/platform_channel_pair.h"
 #include "mojo/edk/embedder/process_delegate.h"
 #include "mojo/edk/system/core.h"
@@ -68,6 +69,10 @@ void SetParentPipeHandleFromCommandLine() {
 }
 
 void Init() {
+  MojoSystemThunks thunks = MakeSystemThunks();
+  size_t expected_size = MojoEmbedderSetSystemThunks(&thunks);
+  DCHECK_EQ(expected_size, sizeof(thunks));
+
   internal::g_core = new Core();
 }