(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 LayerTreeHost::RegisterLayer(Layer* layer) {
   DCHECK(!LayerById(layer->id()));
   DCHECK(!in_paint_layer_contents_);
  layer_id_map_[layer->id()] = layer;
  if (layer->element_id()) {
    mutator_host_->RegisterElement(layer->element_id(),
                                   ElementListType::ACTIVE);
  }
}

 void LayerTreeHost::RegisterLayer(Layer* layer) {
   DCHECK(!LayerById(layer->id()));
   DCHECK(!in_paint_layer_contents_);
  layer_id_map_[layer->id()] = layer;
  if (layer->element_id()) {
    mutator_host_->RegisterElement(layer->element_id(),
                                   ElementListType::ACTIVE);
  }
}
