Check for NULL after Frame lookup in findFrameByName
Check for NULL after Frame lookup in findFrameByName

This function was crashing if the FrameTree name lookup failed to find
a matching frame.

TBR=japhet@chromium.org
BUG=383845

Review URL: https://codereview.chromium.org/331633004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@176058 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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