mac: ensure ui::Compositor exists for visible RWHVs

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 576 matching lines @@
     render_widget_host_->SetView(this);
 
   // Let the page-level input event router know about our surface ID
   // namespace for surface-based hit testing.
   if (render_widget_host_->delegate() &&
       render_widget_host_->delegate()->GetInputEventRouter()) {
     render_widget_host_->delegate()
         ->GetInputEventRouter()
         ->AddSurfaceIdNamespaceOwner(GetSurfaceIdNamespace(), this);
   }
+
+  if (!render_widget_host_->is_hidden())
+    EnsureBrowserCompositorView();
 }
 
 RenderWidgetHostViewMac::~RenderWidgetHostViewMac() {
   display::Screen::GetScreen()->RemoveObserver(this);
 
   // This is being called from |cocoa_view_|'s destructor, so invalidate the
   // pointer.
   cocoa_view_ = nil;
 
   UnlockMouse();
@@ skipping 863 matching lines @@
   if (frame->delegated_frame_data) {
     float scale_factor = frame->metadata.device_scale_factor;
 
     // Compute the frame size based on the root render pass rect size.
     cc::RenderPass* root_pass =
         frame->delegated_frame_data->render_pass_list.back().get();
     gfx::Size pixel_size = root_pass->output_rect.size();
     gfx::Size dip_size = gfx::ConvertSizeToDIP(scale_factor, pixel_size);
 
     root_layer_->SetBounds(gfx::Rect(dip_size));
-    if (!render_widget_host_->is_hidden()) {
-      EnsureBrowserCompositorView();
+    if (!browser_compositor->compositor()) {

[danakj, 2016/05/24 00:34:40]

Is this inverted?

if (!a)
  a->Foo()

is a crash

[ccameron, 2016/05/24 00:37:39]

Just needs "if (browser_compositor)"

       browser_compositor_->compositor()->SetScaleAndSize(
           scale_factor, pixel_size);
     }
 
     SendVSyncParametersToRenderer();
 
     delegated_frame_host_->SwapDelegatedFrame(output_surface_id,
                                               std::move(frame));
   } else {
     DLOG(ERROR) << "Received unexpected frame type.";
@@ skipping 1939 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