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>