android_webview: add to support file uploading

This commit changes function from empty implementation
to abstract function interface in AwContentsClient for
supporting uploading file in browser. Its subclass will
override this function. In Android 4.4, browser is based
on chromium not webkit engine, chromium engine support
uploading file and will callback to client to show file
chooser, but android_webview client has empty
implementation within this callback for file chooser.

So we solve this issue by adding implementation in framework
chromium client which extends android_webview client, function
in framework webkit client and corresponding functions in
browser app.

Change-Id: I50375573848879f3c30069b1831b314f0c536163
chromium: clear favicon bitmap prior to load an url

Favicon bitmap reference should be clear before every call to loadUrl. Otherway, webclient
reports the incorrect favicon. Xe: Aosp browser display the old favicon if the new page
doesn't have a favicon, because webview.getFavicon() returns the old bitmap (about:blank page
shouldn't have to have a favicon).

Patchset 2: Better bug code's description comment

Change-Id: I9c41e3b240c4453092428b895556b94fb938e06f
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>