Add a SharedMainThreadContextProvider method to Platform

This CL is a preparation step to Onion soup webmediaplayer_ms.cc|h
and webmediaplayer_ms_compositor.cc|h
It adds to blink::Platform a methods that expose SharedMainThreadContextProvider()
functionality of RenderThreadImpl needed by WebMediaPlayerMS.

It is part of the phase 4.4 in the design doc [1].

[1] https://docs.google.com/document/d/1rHJGi1U72qZsOGIctIf7GKRaY8d7BgtVW8MaMYumzYY/

BUG=704136
R=guidou@chromium.org, haraken@chromium.org

Change-Id: I3e6534e1a7e362da905ddd78e7e88d51adb5e6eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693241
Commit-Queue: Antonio Gomes (use @igalia instead) <tonikitoo@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676201}

content/renderer/renderer_blink_platform_impl.cc

@@ -541,6 +541,11 @@ RendererBlinkPlatformImpl::NewAudioCapturerSource(
       RenderFrame::GetRoutingIdForWebFrame(web_frame), params);
 }
 
+viz::ContextProvider*
+RendererBlinkPlatformImpl::SharedMainThreadContextProvider() {
+  return RenderThreadImpl::current()->SharedMainThreadContextProvider().get();
+}
+
 //------------------------------------------------------------------------------
 
 std::unique_ptr<WebRTCPeerConnectionHandler>

content/renderer/renderer_blink_platform_impl.h

@@ -127,6 +127,7 @@ class CONTENT_EXPORT RendererBlinkPlatformImpl : public BlinkPlatformImpl {
   scoped_refptr<media::AudioCapturerSource> NewAudioCapturerSource(
       blink::WebLocalFrame* web_frame,
       const media::AudioSourceParameters& params) override;
+  viz::ContextProvider* SharedMainThreadContextProvider() override;
   std::unique_ptr<blink::WebRTCPeerConnectionHandler>
   CreateRTCPeerConnectionHandler(
       blink::WebRTCPeerConnectionHandlerClient* client,

third_party/blink/public/platform/platform.h

@@ -100,6 +100,10 @@ template <class T>
 class Local;
 }
 
+namespace viz {
+class ContextProvider;
+}
+
 namespace webrtc {
 struct RtpCapabilities;
 class AsyncResolverFactory;
@@ -554,6 +558,10 @@ class BLINK_PLATFORM_EXPORT Platform {
     return nullptr;
   }
 
+  virtual viz::ContextProvider* SharedMainThreadContextProvider() {
+    return nullptr;
+  }
+
   // WebRTC ----------------------------------------------------------
 
   // Creates a WebRTCPeerConnectionHandler for RTCPeerConnection.