Convert OfflinePageTabObserver to be per activity.

This observer currently leaks the last visible activity
potentially increasing the chances of Chrome being GC'd.

This should just track each activity separately and be
more explicit about the actions each tab takes.

BUG=711056

Review-Url: https://codereview.chromium.org/2815213002
Cr-Original-Commit-Position: refs/heads/master@{#464748}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2c8b8311c920b0a7beef28c09a11a6f14abfbabc
1 file changed