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::FromID(int render_process_id, int render_view_id) {
  content::RenderViewHost* rvh =
      content::RenderViewHost::FromID(render_process_id, render_view_id);
  if (!rvh) return NULL;
  content::WebContents* web_contents =
      content::WebContents::FromRenderViewHost(rvh);
  if (!web_contents) return NULL;
  return FromWebContents(web_contents);
}

AwContents* AwContents::FromID(int render_process_id, int render_view_id) {
  content::RenderViewHost* rvh =
      content::RenderViewHost::FromID(render_process_id, render_view_id);
  if (!rvh) return NULL;
  content::WebContents* web_contents =
      content::WebContents::FromRenderViewHost(rvh);
  if (!web_contents) return NULL;
  return FromWebContents(web_contents);
}
