Check for RecyclableCompositorMac before accessing its ui::Compositor.

Bug: 833234
Change-Id: I814fc6c736f2fb89cbe32a336fd32a97ac4f4f28
Reviewed-on: https://chromium-review.googlesource.com/1014353Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Ria Jiang <riajiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551081}

content/browser/renderer_host/browser_compositor_view_mac.mm

@@ -233,7 +233,7 @@ void BrowserCompositorMac::ClearCompositorFrame() {
 }
 
 viz::FrameSinkId BrowserCompositorMac::GetRootFrameSinkId() {
-  if (recyclable_compositor_->compositor())
+  if (recyclable_compositor_)
     return recyclable_compositor_->compositor()->frame_sink_id();
   return viz::FrameSinkId();
 }