mac: Fix BookmarkBarControllerTest.TestClearOnDealloc on Yosemite.

In Yosemite, the owner of a nib file is retain/autoreleased during the
initialization of the nib. I wrapped the constructor of the
BookmarkBarController in an @autoreleasepool block to allow the test to control
the destruction timing of the BookmarkBarController.

BUG=390276

Review URL: https://codereview.chromium.org/660063004

Cr-Commit-Position: refs/heads/master@{#300312}

chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller_unittest.mm

@@ -348,12 +348,17 @@ virtual void SetUp() override {
     ASSERT_TRUE(browser());
     AddCommandLineSwitches();
 
-    bar_.reset(
-      [[BookmarkBarControllerNoOpen alloc]
+    // In OSX 10.10, the owner of a nib file is retain/autoreleased during the
+    // initialization of the nib. Wrapping the constructor in an
+    // autoreleasepool ensures that tests can control the destruction timing of
+    // |bar_|.
+    @autoreleasepool {
+      bar_.reset([[BookmarkBarControllerNoOpen alloc]
           initWithBrowser:browser()
              initialWidth:NSWidth([parent_view_ frame])
                  delegate:nil
            resizeDelegate:resizeDelegate_.get()]);
+    }
 
     InstallAndToggleBar(bar_.get());
   }