Fix OOPI crash in extensions::ContentWatcher.
Fix OOPI crash in extensions::ContentWatcher.

extensions::ContentWatcher should observe changes at the frame level,
but for now, just skip remote frames.

BUG=586077

Review URL: https://codereview.chromium.org/1747573002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378360}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk