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AwContentsClientBridgeBase was a browser/ layer interface
for AwContentsClientBridge, since we can make jni calls from
browser/ already, we don't need this interface anymore.

This CL removes AwContentsClientBridgeBase, and put code into
AwContentsClientBridge.

BUG=716604

Review-Url: https://codereview.chromium.org/2888623004
Cr-Commit-Position: refs/heads/master@{#472307}