Enable posting a message from JS to Android webview.

android_webview/browser/aw_browser_context.cc

Index: android_webview/browser/aw_browser_context.cc
diff --git a/android_webview/browser/aw_browser_context.cc b/android_webview/browser/aw_browser_context.cc
index 1fea722a29f1ffb92ef83aba90d1711f5bfe650c..cd620c6f5d3ed15ba04741747e826d119e022255 100644
--- a/android_webview/browser/aw_browser_context.cc
+++ b/android_webview/browser/aw_browser_context.cc
@@ -237,6 +237,14 @@ AwURLRequestContextGetter* AwBrowserContext::GetAwURLRequestContext() {
   return url_request_context_getter_.get();
 }
 
+AwMessagePortService* AwBrowserContext::GetMessagePortService() {
+  if (!message_port_service_.get()) {
+    message_port_service_.reset(
+        native_factory_->CreateAwMessagePortService());
+  }
+  return message_port_service_.get();
+}
+
 // Create user pref service for autofill functionality.
 void AwBrowserContext::CreateUserPrefServiceIfNecessary() {
   if (user_pref_service_)