Implement proper synchronization between HW video decode IPC and CommandBuffer.

gpu/command_buffer/common/command_buffer.h

Index: gpu/command_buffer/common/command_buffer.h
diff --git a/gpu/command_buffer/common/command_buffer.h b/gpu/command_buffer/common/command_buffer.h
index d438f7225acf9762af4a4b6e31cc78aea7deef61..d5fb24d99cff6cf6d01806e512a99c063d054156 100644
--- a/gpu/command_buffer/common/command_buffer.h
+++ b/gpu/command_buffer/common/command_buffer.h
@@ -123,6 +123,27 @@ class CommandBuffer {
   DISALLOW_COPY_AND_ASSIGN(CommandBuffer);
 };
 
+// Synchronizing other mechanisms (such as IPC) with the CommandBuffer requires
+// inserting (writing) a token into the buffer and knowing what the last token
+// read at that point was.  ReadWriteTokens is a convenience struct for passing
+// these pairs around.  Expected usage is to compare a current token to
+// [last_token_read,last_token_written).
+class ReadWriteTokens {
+ public:
+  ReadWriteTokens(int32 read, int32 written);
+  // Required to support pickling.  Use by anything else will DCHECK in InRange.
+  ReadWriteTokens();
+
+  // Return true iff |value| is in the range described by |tokens|, accounting
+  // for (up to) one wrap-around.
+  bool InRange(int32 token) const;
+
+  // These want to be private (and const) but can't in order to support
+  // pickling.
+  int32 last_token_read;
+  int32 last_token_written;
+};
+
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_H_