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 016f35e30e6366be7943585da64800006ec944b5..2fc7e86cb7969e5fe22f219f07fc841ccf48d8e6 100644
--- a/android_webview/browser/aw_browser_context.cc
+++ b/android_webview/browser/aw_browser_context.cc
@@ -240,6 +240,14 @@ 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_)