(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}
bool LayerTreeHostImpl::HaveRootScrollLayer() const {
  return !!InnerViewportScrollLayer();
}

bool LayerTreeHostImpl::HaveRootScrollLayer() const {
  return !!InnerViewportScrollLayer();
}
