Prevent implicit framebuffer clear from clobbering alpha in emulated RGB

content/test/gpu/gpu_tests/pixel_test_pages.py

 # Copyright 2016 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.
 
 class PixelTestPage(object):
   """A wrapper class mimicking the functionality of the PixelTestsStorySet
   from the old-style GPU tests.
   """
   def __init__(self, url, name, test_rect, revision,
                expected_colors=None, tolerance=2, browser_args=None):
@@ skipping 67 matching lines @@
       test_rect=[0, 0, 300, 300],
       revision=1),
 
     PixelTestPage(
       'pixel_webgl_noaa_noalpha.html',
       base_name + '_WebGLGreenTriangle_NoAA_NoAlpha',
       test_rect=[0, 0, 300, 300],
       revision=1),
 
     PixelTestPage(
+      'pixel_webgl_noalpha_implicit_clear.html',
+      base_name + '_WebGLTransparentGreenTriangle_NoAlpha_ImplicitClear',
+      test_rect=[0, 0, 300, 300],
+      revision=1),
+
+    PixelTestPage(
       'pixel_scissor.html',
       base_name + '_ScissorTestWithPreserveDrawingBuffer',
       test_rect=[0, 0, 300, 300],
       revision=0, # This is not used.
       expected_colors=[
         {
           'comment': 'red top',
           'location': [1, 1],
           'size': [198, 188],
           'color': [255, 0, 0],
@@ skipping 201 matching lines @@
       test_rect=[0, 0, 300, 300],
       revision=3),
     PixelTestPage(
       'filter_effects.html',
       base_name + '_CSSFilterEffects_NoOverlays',
       test_rect=[0, 0, 300, 300],
       revision=3,
       tolerance=10,
       browser_args=['--disable-mac-overlays']),
   ]