Add pixel test for solid color backgrounds.

content/test/gpu/page_sets/pixel_tests.py

Index: content/test/gpu/page_sets/pixel_tests.py
diff --git a/content/test/gpu/page_sets/pixel_tests.py b/content/test/gpu/page_sets/pixel_tests.py
index abd458eb83f873eddfb8e08009695a3811ed96e4..e95007fe858b2b243cd51a34e08546e17f6ac67e 100644
--- a/content/test/gpu/page_sets/pixel_tests.py
+++ b/content/test/gpu/page_sets/pixel_tests.py
@@ -199,13 +199,22 @@ class PixelTestsStorySet(story_set_module.StorySet):
 
     self.AddStory(PixelTestsPage(
       url='file://../../data/gpu/pixel_canvas2d_webgl.html',
-      name=base_name + '.2DCanvasWebGL',
+      name=base_name + '.2DCanvasWebGL' + es3_suffix,
       test_rect=[0, 0, 300, 300],
       revision=2,
       story_set=self,
       shared_page_state_class=shared_page_state_class,
       expectations=expectations))
 
+    self.AddStory(PixelTestsPage(
+      url='file://../../data/gpu/pixel_background.html',
+      name=base_name + '.SolidColorBackground' + es3_suffix,
+      test_rect=[500, 500, 100, 100],
+      revision=1,
+      story_set=self,
+      shared_page_state_class=shared_page_state_class,
+      expectations=expectations))
+
   @property
   def allow_mixed_story_states(self):
     # Return True here in order to be able to add the same tests with