Introduce unregisterProtocolHandler() and isProtocolHandlerRegistered() webview API

As a step to support the custom scheme handler in experimental features, this cl introduce unregisterProtocolHandler() and isProtocolHandlerRegistered(). The content module will be able to deregister existing registered protocol and check if a protocol is registered.

BUG=N/A

Review URL: https://codereview.chromium.org/18179009

git-svn-id: svn://svn.chromium.org/blink/trunk@165276 bbb929c8-8fbe-4397-9dbb-9b2b20218538