b/2511635 Browser displays incorrect SSL cert information

WebView's mCertificate member was not cleared when going to new
pages. Rather than clearing mCertificate as was done in previously in
WebView.goBackOrForward, we now clear when CallbackProxy receives a
PAGE_STARTED message.

This problem was highlighted whenever we went to a https page that was
in the cache, since the cache does not store certificate information,
so "More > Page Info > View Certifcate" was showing the certificate of
the last non-cached page because it had not been cleared. See also
b/2516638 "SslCertificate information not cached by CacheManager"

Change-Id: I40284f22ceb7150a6b20ecc2741f6153ed9a3276