Explicitly hold onto objects that are required for migration

Explicitly hold onto objects that are required for migration

The TabContentManager was being garbage collected before migration
finished, resulting in native background threads attempting to
execute JNI on the dead TabContentManager.

More explicitly hold the TabContentManager inside of the FaviconImageCallback
(even though it was already doing so) as a member field instead, which seems
to prevent the GC from nuking it.

Possible reasoning on why the GC was being irrationally exuberant:
http://stackoverflow.com/questions/26642153/finalize-called-on-strongly-reachable-object-in-java-8

BUG=513130

Review URL: https://codereview.chromium.org/1260043007

Cr-Commit-Position: refs/heads/master@{#341971}