Implement PostMessage API for Webview

Bug: 10237116

This CL implements the postmessage API for webview. It provides
a way for java applications to post messages to JS frames and
establish message channels, and then do full-duplex communication
after that.

Change-Id: Idbc2918552a4c9fd28ae1ddf19bd360e71f8a4a2
6 files changed