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These helpers are intended for use with WebContents hosted in tabs, but are
occasionally used outside of that context. This simple fix stops the helper
from exploding in that case.

BUG=849796

Change-Id: I85caef9cd397eec5fd77b8ad16bdcc9d6f3adbbd
Reviewed-on: https://chromium-review.googlesource.com/1089294Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565332}