Allow Blink to generate cc::FrameSinkId from RenderThread

This CL allows Blink to use generate cc::FrameSinkId, internally
implemented using RenderThread's functions.

Currently, Mus is working on making FrameSinkId being able to
generated on Renderer process, getting rid of the need of IPC
to browser just to get a FrameSinkId.

By making this generation function available in Blink, this CL
prepares for a refactoring effort on the current
OffscreenCanvas.commit() workflow--it would facilitate
simplification on CanvasSurfaceLayerBridge
that will remove the current sync IPC that relies on the
browser process to generate  FrameSinkId (which is part of
SurfaceId) for it.

BUG=664547

Review-Url: https://codereview.chromium.org/2514323006
Cr-Commit-Position: refs/heads/master@{#434535}

content/renderer/renderer_blink_platform_impl.cc

@@ -618,6 +618,11 @@ WebString RendererBlinkPlatformImpl::databaseCreateOriginIdentifier(
       WebSecurityOriginToGURL(origin)));
 }
 
+cc::FrameSinkId RendererBlinkPlatformImpl::generateFrameSinkId() {
+  return cc::FrameSinkId(RenderThread::Get()->GetClientId(),
+                         RenderThread::Get()->GenerateRoutingID());
+}
+
 bool RendererBlinkPlatformImpl::isThreadedCompositingEnabled() {
   RenderThreadImpl* thread = RenderThreadImpl::current();
   // thread can be NULL in tests.

content/renderer/renderer_blink_platform_impl.h

@@ -114,6 +114,7 @@ class CONTENT_EXPORT RendererBlinkPlatformImpl : public BlinkPlatformImpl {
                            long long size) override;
   blink::WebString databaseCreateOriginIdentifier(
       const blink::WebSecurityOrigin& origin) override;
+  cc::FrameSinkId generateFrameSinkId() override;
 
   blink::WebString signedPublicKeyAndChallengeString(
       unsigned key_size_index,

third_party/WebKit/public/platform/Platform.h

@@ -55,6 +55,7 @@
 #include "WebVector.h"
 #include "base/metrics/user_metrics_action.h"
 #include "cc/resources/shared_bitmap.h"
+#include "cc/surfaces/frame_sink_id.h"
 
 class GrContext;
 
@@ -321,6 +322,9 @@ class BLINK_PLATFORM_EXPORT Platform {
   // the process's process ID.
   virtual uint32_t getUniqueIdForProcess() { return 0; }
 
+  // Returns a unique FrameSinkID for the current renderer process
+  virtual cc::FrameSinkId generateFrameSinkId() { return cc::FrameSinkId(); }
+
   // Message Ports -------------------------------------------------------
 
   // Creates a Message Port Channel pair. This can be called on any thread.