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

cc/output/begin_frame_args.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 #define CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 
 #include <stdint.h>
 #include <memory>
 
@@ skipping 32 matching lines @@
   enum BeginFrameArgsType {
     INVALID,
     NORMAL,
     MISSED,
     // Not a real type, but used by the IPC system. Should always remain the
     // *last* value in this enum.
     BEGIN_FRAME_ARGS_TYPE_MAX,
   };
   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;
 
   // Creates an invalid set of values.
   BeginFrameArgs();
 
 #ifdef NDEBUG
   typedef const void* CreationLocation;
 #else
   typedef const tracked_objects::Location& CreationLocation;
@@ skipping 100 matching lines @@
   uint32_t remaining_frames;
 
   // |true| if the observer has produced damage (e.g. sent a CompositorFrame or
   // damaged a surface) as part of responding to the BeginFrame.
   bool has_damage;
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_BEGIN_FRAME_ARGS_H_