Don't throw an exception in Apps v2 at start-up.
Don't throw an exception in Apps v2 at start-up.

chrome.tabs is not available in Apps v2, but the functionality we use it for
isn't needed anyway. Rather than throwing a null-pointer exception, log an
error message to remind us to remove the code and associated resources once
we're fully Apps v2.

BUG=121771


Review URL: https://chromiumcodereview.appspot.com/12256043

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@182627 0039d316-1c4b-4281-b951-d872f2087c98

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