Pull the Activity context from WindowAndroid if possible

Pull the Activity context from WindowAndroid if possible

WebView uses ContentViewCore's context for displaying dialogs and
wraps it at construction time. This change makes sure that if there
is an activity that can be reached through WindowAndroid, we use
that as the context inside ColorChooser. Also make sure if there is no
activity, we
dont attempt to create a color chooser and try to show it.
Update ActivityWindowAndroid to take a Context type so
that in Android WebView, the activity context can remain wrapped.

BUG=550410, 570429