Fix a crash in WebViewImpl::copyImageAt.
Fix a crash in WebViewImpl::copyImageAt.

We need null check because hit result may be null for various reasons.

BUG=440894

Review URL: https://codereview.chromium.org/836283002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@188373 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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