Revert of Disable Precompile-platform with clang-cl. (patchset chromi…

…um#1 id:1 of https://codereview.chromium.org/2520833005/ )

Reason for revert:
The crash is now hopefully fixed in clang-cl.

Original issue's description:
> Disable Precompile-platform with clang-cl.
>
> It currently asserts while trying to build that PCH file.
>
> BUG=667891,495697
> NOTRY=true
>
> Committed: https://crrev.com/5b7b04e7fa59d53b8b7917d4c0cf4081abdfb0e3
> Cr-Commit-Position: refs/heads/master@{#434028}

[email protected]
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=667891,495697

Review-Url: https://codereview.chromium.org/2526383002
Cr-Commit-Position: refs/heads/master@{#434534}

third_party/WebKit/Source/core/BUILD.gn

@@ -52,8 +52,7 @@ import("//build/config/pch.gni")
 
 config("blink_core_pch") {
   if (enable_precompiled_headers) {
-    # TODO(thakis): Enable for clang once it works, https://crbug.com/667891
-    if (is_win && !is_clang) {
+    if (is_win) {
       # This is a string rather than a file GN knows about. It has to match
       # exactly what's in the /FI flag below, and what might appear in the
       # source code in quotes for an #include directive.