(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
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}
  void CallOnDraw() {
    if (!TestEnded()) {
      bool resourceless_software_draw = false;
      ImplThreadTaskRunner()->PostTask(
          FROM_HERE, base::Bind(&OnDrawCompositorFrameSink::OnDraw,
                                base::Unretained(compositor_frame_sink_),
                                resourceless_software_draw));
    }
  }

  void CallOnDraw() {
    if (!TestEnded()) {
      bool resourceless_software_draw = false;
      ImplThreadTaskRunner()->PostTask(
          FROM_HERE, base::Bind(&OnDrawCompositorFrameSink::OnDraw,
                                base::Unretained(compositor_frame_sink_),
                                resourceless_software_draw));
    }
  }
