Reland fix for thumbnail generation.

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 e4a9f42be8091d18bd02a8a15b805fa84da4d6fc..df1948626cdd5c1956dc141beb7d575246d356f2 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -655,10 +655,12 @@ void RenderWidgetHostViewMac::DestroyBrowserCompositorView() {
 
   // Destroy the BrowserCompositorView to transition Suspended -> Destroyed.
   if (browser_compositor_state_ == BrowserCompositorSuspended) {
-    browser_compositor_->accelerated_widget_mac()->ResetNSView();
-    browser_compositor_->compositor()->SetScaleAndSize(1.0, gfx::Size(0, 0));
-    browser_compositor_->compositor()->SetRootLayer(nullptr);
-    BrowserCompositorMac::Recycle(browser_compositor_.Pass());
+    if (browser_compositor_) {
+      browser_compositor_->accelerated_widget_mac()->ResetNSView();
+      browser_compositor_->compositor()->SetScaleAndSize(1.0, gfx::Size(0, 0));
+      browser_compositor_->compositor()->SetRootLayer(nullptr);
+      BrowserCompositorMac::Recycle(std::move(browser_compositor_));
+    }
     browser_compositor_state_ = BrowserCompositorDestroyed;
   }
 }
@@ -893,7 +895,7 @@ void RenderWidgetHostViewMac::WasOccluded() {
   // occur in this specific order. However, because thumbnail generation is
   // asychronous, that operation won't run before SuspendBrowserCompositorView()
   // completes. As a result you won't get a thumbnail for the page unless you
-  // happen to switch back to it. See http://crbug.com/530707 .
+  // execute these two statements in this specific order.
   render_widget_host_->WasHidden();
   SuspendBrowserCompositorView();
 }