Allow ui::Compositor to disable commits during tab-switch

content/browser/renderer_host/render_widget_host_view_mac.mm

Index: content/browser/renderer_host/render_widget_host_view_mac.mm
diff --git a/content/browser/renderer_host/render_widget_host_view_mac.mm b/content/browser/renderer_host/render_widget_host_view_mac.mm
index 7f3cb354dc05c946fc6ce1534b5eb6a1ebcd1fea..1ad6b78dbe409bb5bd4564ffccd12eb7738d33be 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -615,6 +615,14 @@ void RenderWidgetHostViewMac::EnsureBrowserCompositorView() {
   if (browser_compositor_state_ == BrowserCompositorSuspended) {
     delegated_frame_host_->SetCompositor(browser_compositor_->compositor());
     delegated_frame_host_->WasShown(ui::LatencyInfo());
+    // Unsuspend the browser compositor after showing the delegated frame host.
+    // If there is not a saved delegated frame, then the delegated frame host
+    // will keep the compositor locked until a delegated frame is swapped.
+    float scale_factor = ViewScaleFactor();
+    browser_compositor_->compositor()->SetScaleAndSize(
+        scale_factor,
+        gfx::ConvertSizeToPixel(scale_factor, GetViewBounds().size()));
+    browser_compositor_->Unsuspend();
     browser_compositor_state_ = BrowserCompositorActive;
   }
 }
@@ -625,6 +633,9 @@ void RenderWidgetHostViewMac::SuspendBrowserCompositorView() {
 
   // Hide the DelegatedFrameHost to transition from Active -> Suspended.
   if (browser_compositor_state_ == BrowserCompositorActive) {
+    // Ensure that any changes made to the ui::Compositor do not result in new
+    // frames being produced.
+    browser_compositor_->Suspend();
     // Marking the DelegatedFrameHost as removed from the window hierarchy is
     // necessary to remove all connections to its old ui::Compositor.
     delegated_frame_host_->WasHidden();
@@ -834,6 +845,7 @@ void RenderWidgetHostViewMac::Show() {
   if (!render_widget_host_->is_hidden())
     return;
 
+  EnsureBrowserCompositorView();

[danakj, 2015/03/12 00:07:15]

How come this is needed?

[ccameron, 2015/03/12 01:01:34]

Oh -- I added the explanation to the CL description, but to any comments:

  Also, re-create the browser compositor immediately after the tab is made
  visible. This way, if the DelegatedFrameHost has an un-evicted old frame,
  then that frame will be displayed immediately (and if not, the compositor
  will remain locked until a new frame is swapped in).

Also added a comment to this effect.

[danakj, 2015/03/12 01:08:50]

Thanks :)

   WasUnOccluded();
 
   // If there is not a frame being currently drawn, kick one, so that the below