Implement android.webkit.BrowserFrame.requestClientCert

Following the example of reportSslCertError, implement requestClientCert

ERROR CASE                                              CLIENT CERT CASE
<... From external/webkit ...>                          <... From external/webkit ...>
android.webkit.BrowserFrame.reportSslCertError          android.webkit.BrowserFrame.requestClientCert
CallbackProxy.onReceivedSslError                        CallbackProxy.onReceivedClientCertRequest
WebViewClient.onReceivedSslError                        WebViewClient.onReceivedClientCertRequest
<... See packages/apps/Browser ...>                     <... See packages/apps/Browser ...>
SslErrorHandler.proceed (with SslCertLookupTable)       ClientCertRequestHandler.proceed (with SslClientCertLookupTable)
android.webkit.BrowserFrame.nativeSslCertErrorProceed   android.webkit.BrowserFrame.nativeSslClientCert
<... To external/webkit ...>                            <... To external/webkit ...>

Change-Id: I2ba6007ad9b2ee520a0a6b17f3a767679b1664de