[Merge M48] mac: Fix unit test crash.

> The test OmniboxPopupViewMacTest was crashing because of an invalid pointer
> dereference. The root cause was that a base::scoped_nsobject member was captured
> as an implicit parameter to an Obj-C block, which makes a raw copy of the |this|
> pointer. I changed the captured parameter to be an Obj-C object, which
> automatically gets retained/released.
>
> BUG=551007, 560482
>
> Review URL: https://codereview.chromium.org/1466333004
>
> Cr-Commit-Position: refs/heads/master@{#362184}

(cherry picked from commit 36c234e)
TBR=mark@chromium.org
BUG=560482

Review URL: https://codereview.chromium.org/1486393002 .

Cr-Commit-Position: refs/branch-heads/2564@{#194}
Cr-Branched-From: 1283eca-refs/heads/master@{#359700}

chrome/browser/ui/cocoa/omnibox/omnibox_popup_view_mac.mm

@@ -284,8 +284,9 @@
   // |popup_| gets redrawn once the animation completes. See
   // http://crbug.com/538590 and http://crbug.com/551007 .
   if (base::mac::IsOSElCapitanOrLater()) {
+    NSWindow* popup = popup_.get();
     [[NSAnimationContext currentContext] setCompletionHandler:^{
-      [popup_ display];
+      [popup display];
     }];
   }
   [[popup_ animator] setFrame:popup_frame display:YES];