InMemory MediaCapabilities DB for Guest/Incognito profiles

Prior to this change, Guest mode has an empty read-only DB and Incongito
has a read-only DB. This CL allows both profiles to write to an "in
memory" database that disapears when the profile is destroyed.

This improves the experience for the user and avoids the possibility
of using MediaCapabilites to detect incognito/guest profiles.

R=jam@chromium.org

Bug: 796280
Change-Id: I37e25478ce6efe8c25bf49f77929f94c9ca27655
Reviewed-on: https://chromium-review.googlesource.com/1092156
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569046}
17 files changed