Add a unit test hook into WebURLLoaderClient callbacks.
Add a unit test hook into WebURLLoaderClient callbacks.

Once the Chrome-side patch is landed too, this allows unit tests
fine-grained control over how and when callbacks for receiving data are
invoked.

BUG=515876

Review URL: https://codereview.chromium.org/1272293006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200266 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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