Implement external stencil for Android WebView

cc/output/renderer_pixeltest.cc

Index: cc/output/renderer_pixeltest.cc
diff --git a/cc/output/renderer_pixeltest.cc b/cc/output/renderer_pixeltest.cc
index 6c5e255822d5b5a5c47710752201aa1981465e66..f5afaea34c48ba05a9c42231a312111a042ec9e2 100644
--- a/cc/output/renderer_pixeltest.cc
+++ b/cc/output/renderer_pixeltest.cc
@@ -1889,6 +1889,7 @@ class ExternalStencilPixelTest : public GLRendererPixelTest {
                 device_viewport_size_.width(),
                 device_viewport_size_.height());
     gl->Clear(GL_STENCIL_BUFFER_BIT);
+    gl->StencilFunc(GL_EQUAL, 1, 1);
   }
 };