[AppCache] Use UnguessableToken for appcache host IDs.

Rather than having the browser and renderer processes try to allocate
non conflicting integers, and keying everything of process id +
host id, instead identify all appcache hosts by a unguessable token.

This hopefully fixes some race conditions when a process id gets
reused for a new process before the old state has fully torn down, and
should make the code slightly more robust against compromised renderers
trying to claim pre-registered browser created AppCacheHosts that were
meant for a different process/frame.

Bug: 963661
Change-Id: I4fbf1835c3ca22ee26e7d1b9a6bd142d0152465d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1620697
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662283}
67 files changed