MacViews: Make window content views layer-backed.

ui/views/cocoa/native_widget_mac_nswindow.mm

Index: ui/views/cocoa/native_widget_mac_nswindow.mm
diff --git a/ui/views/cocoa/native_widget_mac_nswindow.mm b/ui/views/cocoa/native_widget_mac_nswindow.mm
index 0f077fe12b65c90ba4126e6ba6496e134b0071f7..6bd782a6a4d50596604ae490ab8c6b628e87c4d7 100644
--- a/ui/views/cocoa/native_widget_mac_nswindow.mm
+++ b/ui/views/cocoa/native_widget_mac_nswindow.mm
@@ -41,6 +41,7 @@
                                styleMask:windowStyle
                                  backing:bufferingType
                                    defer:deferCreation])) {
+    static_cast<NSView*>(self.contentView).wantsLayer = YES;

[tapted, 2017/01/13 00:22:37]

ooh neat - I think this might fix http://crbug.com/649354 ? (wdyt - I'm not sure
if I can repro that.. but if(yes) can you cite that bug here?)  At the very
least it fixes the task manager not having rounded corners, and some resize
artifacts related to that \o/ - I can make a crbug for that if 649354 doesn't
fit.

But I think there's a downside -- this will need to allocate an additional
window-sized texture. So, I think there's no point doing this for fully
transparent windows. The best place for this might be in
bridged_native_widget.mm, with

  } else {
    [[window_ contentView] setWantsLayer:YES];
  }

towards the end of BridgedNativeWidget::CreateLayer()

This will happen after BridgedNativeWidget::SetRootView(..) which may replace
the windows' contentView.


Another approach might be to check for NSTexturedBackgroundWindowMask since
they're the ones AppKit promises to give rounded bottom corners to.

(but in either case, I think BridgedNativeWidget::CreateLayer is the right place
for it -- it seems like a quirk that this manages to work, since I'm pretty sure
SetRootView() happens after the initializer here, and it should be replacing the
contentView with a new one)

     commandDispatcher_.reset([[CommandDispatcher alloc] initWithOwner:self]);
   }
   return self;