Implement external stencil for Android WebView

cc/output/renderer_pixeltest.cc

 // Copyright 2012 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.
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/message_loop/message_loop.h"
 #include "cc/output/gl_renderer.h"
 #include "cc/quads/draw_quad.h"
@@ skipping 1871 matching lines @@
     gl->Scissor(0,
                 0,
                 device_viewport_size_.width() / 2,
                 device_viewport_size_.height() / 2);
     gl->Clear(GL_STENCIL_BUFFER_BIT);
     gl->Scissor(device_viewport_size_.width() / 2,
                 device_viewport_size_.height() / 2,
                 device_viewport_size_.width(),
                 device_viewport_size_.height());
     gl->Clear(GL_STENCIL_BUFFER_BIT);
+    gl->StencilFunc(GL_EQUAL, 1, 1);
   }
 };
 
 TEST_F(ExternalStencilPixelTest, StencilTestEnabled) {
   ClearBackgroundToGreen();
   PopulateStencilBuffer();
   this->EnableExternalStencilTest();
 
   // Draw a blue quad that covers the entire device viewport. It should be
   // clipped to the bottom left and top right corners by the external stencil.
@@ skipping 1072 matching lines @@
 
   EXPECT_TRUE(this->RunPixelTest(
       &pass_list, base::FilePath(FILE_PATH_LITERAL("spiral.png")),
       FuzzyPixelOffByOneComparator(true)));
 }
 
 #endif  // !defined(OS_ANDROID)
 
 }  // namespace
 }  // namespace cc