Don't register JNI methods for the android_webview.

Rather than registering all jni bindings at startup, only get references
to the class object for those files which require bindings. All others
are satisfied by exporting symbols which can be found automatically by
the VM.

BUG=402003

Review URL: https://codereview.chromium.org/454923002

git-svn-id: http://src.chromium.org/svn/trunk/src/base@288685 4ff67af0-8c30-449e-8e8b-ad334ec8d88c