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This CL prefixes the DidFinishLoading and DidReceiveData methods in
BrowserPluginDelegate, so they don't conflict with the
WebAssociatedURLLoaderClient versions which currently start with
lowercase letter. As part of the Blink rewrite to match Chromium style,
those will be colliding in MimeHandlerViewContainer.

BUG=582312

Review-Url: https://codereview.chromium.org/2616123002
Cr-Commit-Position: refs/heads/master@{#441994}