mac: Don't pause for frames with no frame sink

This should not have been removed in https://crrev.com/536430

Move tracking from RenderWidgetHostImpl (where it is tracked only in
non-viz mode) to BrowserCompositorView, where it is tracked in both
modes.

Bug: 772576, 812505, 812156
Change-Id: I006c1c78f2dc148602f08253745993f8a57a7e16
Reviewed-on: https://chromium-review.googlesource.com/923379
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537604}

content/browser/renderer_host/browser_compositor_view_mac.mm

@@ -524,6 +524,11 @@ void BrowserCompositorMac::EndPauseForFrame() {
 }
 
 bool BrowserCompositorMac::ShouldContinueToPauseForFrame() const {
+  // The renderer won't produce a frame if its frame sink hasn't been created
+  // yet.
+  if (!renderer_compositor_frame_sink_)
+    return false;
+
   if (!recyclable_compositor_)
     return false;