Disable flash by default for M76.

This sets the default flash content setting to block and enables an
already-prepared information banner that warns users who override the
setting that flash will stop being supported in December 2020.

Bug: 882276
Change-Id: Ifa27c0462abc1e17bb5309b8eea187997096aa9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585882
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Anthony LaForge <laforge@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Kamila Hasanbega <hkamila@google.com>
Cr-Commit-Position: refs/heads/master@{#655960}