Relands the following. First attempt was reverted because it broke

ui/gfx/compositor/compositor.gyp

 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'target_defaults': {
     'sources/': [
@@ skipping 42 matching lines @@
         'layer_animation_delegate.h',
         'layer_animation_element.cc',
         'layer_animation_element.h',
         'layer_animation_observer.h',
         'layer_animation_sequence.cc',
         'layer_animation_sequence.h',
         'layer_animator.cc',
         'layer_animator.h',
         'screen_rotation.cc',
         'screen_rotation.h',
-        # This is needed for ui tests, so it's always compiled.
+        # UI tests need TestWebGraphicsContext3D, so we always build it.
+        'test_web_graphics_context_3d.cc',
         'test_web_graphics_context_3d.h',
       ],
       'conditions': [
         ['os_posix == 1', {
           'sources!': [
             'compositor_stub.cc',
           ],
         }],
         ['OS == "win" and views_compositor == 1', {
           'sources!': [
@@ skipping 134 matching lines @@
             'test/test_compositor.cc',
             'test/test_compositor.h',
             'test/test_texture.cc',
             'test/test_texture.h',
           ],
         }],
       ],
     },
   ],
 }