Move flush id from WebGraphicsContext3DImpl to GLES2Implementation.

gpu/command_buffer/client/gles2_implementation.h

Index: gpu/command_buffer/client/gles2_implementation.h
diff --git a/gpu/command_buffer/client/gles2_implementation.h b/gpu/command_buffer/client/gles2_implementation.h
index 02314d07fd644f1f20f59d3d2e59fd0be10bb4a4..6ee901e2a05cb73a8f56c3556d51403e7ece9349 100644
--- a/gpu/command_buffer/client/gles2_implementation.h
+++ b/gpu/command_buffer/client/gles2_implementation.h
@@ -792,6 +792,9 @@ class GLES2_IMPL_EXPORT GLES2Implementation
   // Used to check for single threaded access.
   int use_count_;
 
+  // Changed every time a flush or finish occurs.
+  uint32_t flush_id_;
+
   // Maximum amount of extra memory from the mapped memory pool to use when
   // needing to transfer something exceeding the default transfer buffer.
   // This should be 0 for low memory devices since they are already memory