command_buffer: Implement CHROMIUM_framebuffer_mixed_samples extension

gpu/skia_bindings/gl_bindings_skia_cmd_buffer.cc

Index: gpu/skia_bindings/gl_bindings_skia_cmd_buffer.cc
diff --git a/gpu/skia_bindings/gl_bindings_skia_cmd_buffer.cc b/gpu/skia_bindings/gl_bindings_skia_cmd_buffer.cc
index 0203d07db2dc2d10e93152b8cf53e81d17d17eb9..09c7c7cd57d9d44cee662244a300cb3ff646be44 100644
--- a/gpu/skia_bindings/gl_bindings_skia_cmd_buffer.cc
+++ b/gpu/skia_bindings/gl_bindings_skia_cmd_buffer.cc
@@ -181,6 +181,7 @@ void InitCommandBufferSkiaGLBinding(GrGLInterface* interface) {
         glProgramPathFragmentInputGenCHROMIUM;
     functions->fBindFragmentInputLocation = glBindFragmentInputLocationCHROMIUM;
   }
+  functions->fCoverageModulation = glCoverageModulationCHROMIUM;
 }
 
 }  // namespace skia