sync compositor: pass simple gfx types by const ref

See bug for reasoning

BUG=159273

Review URL: https://codereview.chromium.org/1417893006

Cr-Commit-Position: refs/heads/master@{#356653}
AwContents::~AwContents() {
  DCHECK_EQ(this, AwContents::FromWebContents(web_contents_.get()));
  web_contents_->RemoveUserData(kAwContentsUserDataKey);
  if (find_helper_.get())
    find_helper_->SetListener(NULL);
  if (icon_helper_.get())
    icon_helper_->SetListener(NULL);
  base::subtle::Atomic32 instance_count =
      base::subtle::NoBarrier_AtomicIncrement(&g_instance_count, -1);
  if (instance_count == 0) {
    base::MemoryPressureListener::NotifyMemoryPressure(
        base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL);
  }
}

AwContents::~AwContents() {
  DCHECK_EQ(this, AwContents::FromWebContents(web_contents_.get()));
  web_contents_->RemoveUserData(kAwContentsUserDataKey);
  if (find_helper_.get())
    find_helper_->SetListener(NULL);
  if (icon_helper_.get())
    icon_helper_->SetListener(NULL);
  base::subtle::Atomic32 instance_count =
      base::subtle::NoBarrier_AtomicIncrement(&g_instance_count, -1);
  if (instance_count == 0) {
    base::MemoryPressureListener::NotifyMemoryPressure(
        base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL);
  }
}
