Fix SharedImageBackingFactoryGLTextureTest.Image on ES2

Part of the test uses R8 textures which are not guaranteed to be
available. Skip that part if they're not.

Bug: 929462
Change-Id: Ia57df90b9d6a997d3de1816f267b135bd8aee482
Reviewed-on: https://chromium-review.googlesource.com/c/1481605
Auto-Submit: Antoine Labour <piman@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634473}

gpu/command_buffer/service/shared_image_backing_factory_gl_texture_unittest.cc

@@ -288,7 +288,8 @@ TEST_P(SharedImageBackingFactoryGLTextureTest, Image) {
   shared_image.reset();
   EXPECT_FALSE(mailbox_manager_.ConsumeTexture(mailbox));
 
-  if (!use_passthrough()) {
+  if (!use_passthrough() &&
+      context_state_->feature_info()->feature_flags().ext_texture_rg) {
     // Create a R-8 image texture, and check that the internal_format is that of
     // the image (GL_RGBA for TextureImageFactory). This only matters for the
     // validating decoder.