cc: Make cc_unittests opt into real GL NullDraw contexts.

cc/test/pixel_test.cc

Index: cc/test/pixel_test.cc
diff --git a/cc/test/pixel_test.cc b/cc/test/pixel_test.cc
index 0a31bb0ccbb659ce6274632afc05bafe66cc9b8f..77a8f523f2091e8d72e08f6a20fbff282cf9b6b5 100644
--- a/cc/test/pixel_test.cc
+++ b/cc/test/pixel_test.cc
@@ -117,6 +117,8 @@ bool PixelTest::PixelsMatchReference(const base::FilePath& ref_file,
 }
 
 void PixelTest::SetUpGLRenderer(bool use_skia_gpu_backend) {
+  enable_pixel_output_.reset(new gfx::DisableNullDrawGLBindings);
+
   output_surface_.reset(
       new PixelTestOutputSurface(new TestInProcessContextProvider));
   output_surface_->BindToClient(output_surface_client_.get());