Reland fix for thumbnail generation.

content/browser/renderer_host/render_widget_host_view_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/render_widget_host_view_mac.h"
 
 #import <objc/runtime.h>
 #include <OpenGL/gl.h>
 #include <QuartzCore/QuartzCore.h>
 #include <stdint.h>
@@ skipping 637 matching lines @@
 
 void RenderWidgetHostViewMac::DestroyBrowserCompositorView() {
   TRACE_EVENT0("browser",
                "RenderWidgetHostViewMac::DestroyBrowserCompositorView");
 
   // Transition from Active -> Suspended if need be.
   SuspendBrowserCompositorView();
 
   // 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;
   }
 }
 
 void RenderWidgetHostViewMac::DestroySuspendedBrowserCompositorViewIfNeeded() {
   if (browser_compositor_state_ != BrowserCompositorSuspended)
     return;
 
   // If this view is in a window that is visible, keep around the suspended
   // BrowserCompositorView in case |cocoa_view_| is suddenly revealed (so that
@@ skipping 214 matching lines @@
 void RenderWidgetHostViewMac::WasOccluded() {
   if (render_widget_host_->is_hidden())
     return;
 
   // Note that the following call to WasHidden() can trigger thumbnail
   // generation on behalf of the NTP, and that cannot succeed if the browser
   // compositor view has been suspended. Therefore these two statements must
   // 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();
 }
 
 void RenderWidgetHostViewMac::SetSize(const gfx::Size& size) {
   gfx::Rect rect = GetViewBounds();
   rect.set_size(size);
   SetBounds(rect);
 }
 
@@ skipping 2528 matching lines @@
 
 // "-webkit-app-region: drag | no-drag" is implemented on Mac by excluding
 // regions that are not draggable. (See ControlRegionView in
 // native_app_window_cocoa.mm). This requires the render host view to be
 // draggable by default.
 - (BOOL)mouseDownCanMoveWindow {
   return YES;
 }
 
 @end