Android: Allow duplicate calls to InitApplicationContext.

Allow InitApplicationContext to be called more than once as long as the
same context is passed each time. This will make it possible for
AwCookieManager to make use of certain JNI functions before Chromium has
been initialised without needing to keep track of whether it's already
passed the application context to native or not.

BUG=304813
NOTRY=true

Review URL: https://codereview.chromium.org/54923002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@234271 4ff67af0-8c30-449e-8e8b-ad334ec8d88c