Fix crash in OfflinePageAutoFetcher

Lifetime of RenderFrameHost was not properly maintained.

Bug: 977462

Change-Id: I66c16fa397a42c778b22bb1ad8909a3b1cf1b41f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679551
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675278}
6 files changed