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How this works is that we create a WebContents in the background to a Drive
endpoint that will take care of all the offline initialization for the app.
We succeed if a background page is opened; otherwise, we fail if the page does
not load or after a timeout.

BUG=307302
TEST=manual

Review URL: https://codereview.chromium.org/55423004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233501 0039d316-1c4b-4281-b951-d872f2087c98