Use a static variable and methods for the current WebView.

sJavaBridge may not exist by the time onWindowFocusChanged is called.
To avoid an NPE, just static methods that modify a global field.
JWebCoreJavaBridge is a global object anyway so using a static field
will be fine.  To avoid any garbage collection issues, store the
WebView in a WeakReference.

Bug: 2894682
Change-Id: I73de9237ddbbdc55841a9b2639437e26856439c8