(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}
ManagedMemoryPolicy LayerTreeHostImpl::ActualManagedMemoryPolicy() const {
  ManagedMemoryPolicy actual = cached_managed_memory_policy_;
  if (debug_state_.rasterize_only_visible_content) {
    actual.priority_cutoff_when_visible =
        gpu::MemoryAllocation::CUTOFF_ALLOW_REQUIRED_ONLY;
  } else if (use_gpu_rasterization()) {
    actual.priority_cutoff_when_visible =
        gpu::MemoryAllocation::CUTOFF_ALLOW_NICE_TO_HAVE;
  }
  return actual;
}

ManagedMemoryPolicy LayerTreeHostImpl::ActualManagedMemoryPolicy() const {
  ManagedMemoryPolicy actual = cached_managed_memory_policy_;
  if (debug_state_.rasterize_only_visible_content) {
    actual.priority_cutoff_when_visible =
        gpu::MemoryAllocation::CUTOFF_ALLOW_REQUIRED_ONLY;
  } else if (use_gpu_rasterization()) {
    actual.priority_cutoff_when_visible =
        gpu::MemoryAllocation::CUTOFF_ALLOW_NICE_TO_HAVE;
  }
  return actual;
}
