Implement external stencil for Android WebView

android_webview/browser/aw_render_thread_context_provider.cc

Index: android_webview/browser/aw_render_thread_context_provider.cc
diff --git a/android_webview/browser/aw_render_thread_context_provider.cc b/android_webview/browser/aw_render_thread_context_provider.cc
index acc6e36f290efc21c1408298a8eba6788a268c51..27dbf714b97bba5d9348f52f5d56fcab8b539c29 100644
--- a/android_webview/browser/aw_render_thread_context_provider.cc
+++ b/android_webview/browser/aw_render_thread_context_provider.cc
@@ -53,7 +53,7 @@ AwRenderThreadContextProvider::AwRenderThreadContextProvider(
   blink::WebGraphicsContext3D::Attributes attributes;
   attributes.antialias = false;
   attributes.depth = false;
-  attributes.stencil = false;
+  attributes.stencil = true;
   attributes.shareResources = true;
   attributes.noAutomaticFlushes = true;
   gpu::gles2::ContextCreationAttribHelper attribs_for_gles2;