Rename getWebView() to getWebViewClassic

Interdepends on https://android-git.corp.google.com/g/170037
Tidys up a TODO from my webview proxy patch.
This allows me to re-introduce getWebView(), but now returning the
actual WebView instance which is needed by my upcoming change to WebKit
linked above.
Also moves sendPluginDrawMsg() to WebViewCore for convenience in the
MediaTexture code that calls this.

Change-Id: I57b72504a792de58d15f386abf4a9d821449ab0a