Fix a bug in drawBuffers optimization.

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index 3c2f69440091f8020df96bec44979fa8f6892b18..5739660354ae36f4a2fb7454d04734c2476d03e5 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -1337,6 +1337,10 @@ class GLES2DecoderImpl : public GLES2Decoder, public ErrorStateClient {
   // NONE through DrawBuffers, to be on the safe side. Return true.
   bool ValidateAndAdjustDrawBuffers(const char* function_name);
 
+  // Filter out the draw buffers that have no images attached but are not NONE
+  // through DrawBuffers, to be on the safe side.
+  void AdjustDrawBuffers();
+
   // Checks if all active uniform blocks in the current program are backed by
   // a buffer of sufficient size.
   // If not, generates an INVALID_OPERATION to avoid undefined behavior in
@@ -7287,6 +7291,7 @@ error::Error GLES2DecoderImpl::DoClear(GLbitfield mask) {
         return error::kNoError;
       }
     }
+    AdjustDrawBuffers();
     glClear(mask);
   }
   return error::kNoError;
@@ -8846,6 +8851,16 @@ bool GLES2DecoderImpl::ValidateAndAdjustDrawBuffers(const char* func_name) {
   return true;
 }
 
+void GLES2DecoderImpl::AdjustDrawBuffers() {
+  if (!SupportsDrawBuffers()) {
+    return;
+  }
+  Framebuffer* framebuffer = framebuffer_state_.bound_draw_framebuffer.get();
+  if (framebuffer) {
+    framebuffer->AdjustDrawBuffers();
+  }
+}
+
 bool GLES2DecoderImpl::ValidateUniformBlockBackings(const char* func_name) {
   DCHECK(feature_info_->IsWebGL2OrES3Context());
   if (!state_.current_program.get())