Modify BrowserFrame.requestClientCert() to take the host and port as a String

Currently we get the host and port from Chromium as a std::string, convert to a
jbyte array to pass over JNI, then convert to String. It's simpler to convert
directly to jstring and to pass that over JNI.

Requires https://android-git.corp.google.com/g/141234
in external/webkit.

Bug: 5442710

Change-Id: I5480471a841c24481ef09d836a8b0c778251b119