[cc] Set BeginFrame sequence numbers on CompositorFrames from Scheduler.

cc/output/begin_frame_args.h

Index: cc/output/begin_frame_args.h
diff --git a/cc/output/begin_frame_args.h b/cc/output/begin_frame_args.h
index 7fe704c572be0e822cbe4fa7b2ab2d2bd10a0853..4b508e22e50d28f9bfe0ca4ae43f5eaed39b8e9a 100644
--- a/cc/output/begin_frame_args.h
+++ b/cc/output/begin_frame_args.h
@@ -50,6 +50,12 @@ struct CC_EXPORT BeginFrameArgs {
   };
   static const char* TypeToString(BeginFrameArgsType type);
 
+  static constexpr uint32_t kStartingSourceId = 0;
+  // |source_id| for BeginFrameArgs not created by a BeginFrameSource. Used to
+  // avoid sequence number conflicts of BeginFrameArgs manually fed to an
+  // observer with those fed to the observer by the its BeginFrameSource.
+  static constexpr uint32_t kManualSourceId = UINT32_MAX;
+
   static constexpr uint64_t kInvalidFrameNumber = 0;
   static constexpr uint64_t kStartingFrameNumber = 1;