(Reland) Discard compositor frames from unloaded web content

      This is a reland of https://codereview.chromium.org/2707243005/ with a
small change to fix an uninitialized memory error that fails on MSAN
bots.

BUG=672847,648117
TBR=danakj@chromium.org, creis@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2731283003
Cr-Commit-Position: refs/heads/master@{#454954}
(cherry picked from commit 5d78b84d39bd34bc9fce9d01c0dcd5a22a330d34)

Review-Url: https://codereview.chromium.org/2793013002 .
Cr-Commit-Position: refs/branch-heads/3029@{#547}
Cr-Branched-From: 939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}