Fix a bug in drawBuffers optimization.

gpu/command_buffer/service/framebuffer_manager.h

Index: gpu/command_buffer/service/framebuffer_manager.h
diff --git a/gpu/command_buffer/service/framebuffer_manager.h b/gpu/command_buffer/service/framebuffer_manager.h
index d2ebe070e5a30bf36d6189d89312c6d0cec72dc4..21d4a39c709624e1761635c3b1afbbfcbf78ac1e 100644
--- a/gpu/command_buffer/service/framebuffer_manager.h
+++ b/gpu/command_buffer/service/framebuffer_manager.h
@@ -194,6 +194,11 @@ class GPU_EXPORT Framebuffer : public base::RefCounted<Framebuffer> {
   bool ValidateAndAdjustDrawBuffers(uint32_t fragment_output_type_mask,
                                     uint32_t fragment_output_written_mask);
 
+  // Filter out the draw buffers that have no images attached but are not NONE
+  // through DrawBuffers, to be on the safe side.
+  // This is applied before a clear call.
+  void AdjustDrawBuffers();
+
   bool ContainsActiveIntegerAttachments() const;
 
   // Return true if any draw buffers has an alpha channel.