Add Begin/EndSharedImageAccessCHROMIUM

Adds begin/end functions to the GL command buffer for shared image
access. Currently, begin/end is a no-op in all
SharedImageRepresentations, this will be fixed up in a follow up.

This change just adds the commands and parameter validation in the
validating and passthrough command decoders.

Bug: 891059
Change-Id: Idb4bd900fa56bd470ad44ad955267e76860792cd
Reviewed-on: https://chromium-review.googlesource.com/c/1313668
Commit-Queue: Eric Karl <ericrk@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606030}
diff --git a/gpu/command_buffer/service/texture_manager.h b/gpu/command_buffer/service/texture_manager.h
index ccc612d3..8d5e7b8 100644
--- a/gpu/command_buffer/service/texture_manager.h
+++ b/gpu/command_buffer/service/texture_manager.h
@@ -696,6 +696,9 @@
   GLuint client_id() const { return client_id_; }
   GLuint service_id() const { return texture_->service_id(); }
   GLint num_observers() const { return num_observers_; }
+  SharedImageRepresentationGLTexture* shared_image() const {
+    return shared_image_.get();
+  }
 
   // When the TextureRef is destroyed, it will assume that the context has been
   // lost, regardless of the state of the TextureManager.