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
4ab72cb7e7