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After below CLs, the check is unnecessary, because for linux_chromeos,
if no extension background pages, key events won't be blocked by IMF.

https://codereview.chromium.org/433163005/
https://codereview.chromium.org/444523003/

BUG=None
TEST=Verified on linux_chromeos.

Review URL: https://codereview.chromium.org/462383003

Cr-Commit-Position: refs/heads/master@{#289566}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289566 0039d316-1c4b-4281-b951-d872f2087c98