Fix BridgedNativeWidgetTest test crashes after r379525

ui/views/cocoa/bridged_native_widget_unittest.mm

 // Copyright 2014 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.
 
 #import "ui/views/cocoa/bridged_native_widget.h"
 
 #import <Cocoa/Cocoa.h>
 
 #import "base/mac/foundation_util.h"
 #import "base/mac/mac_util.h"
@@ skipping 261 matching lines @@
   EXPECT_TRUE([window delegate]);
   bridge()->SetRootView(view_.get());
   ns_view_ = bridge()->ns_view();
 
   // Pretend it has been shown via NativeWidgetMac::Show().
   [window orderFront:nil];
   [test_window() makePretendKeyWindowAndSetFirstResponder:bridge()->ns_view()];
 }
 
 void BridgedNativeWidgetTest::TearDown() {
+  if (bridge())
+    bridge()->SetRootView(nullptr);
   view_.reset();
   BridgedNativeWidgetTestBase::TearDown();
 }
 
 // The TEST_VIEW macro expects the view it's testing to have a superview. In
 // these tests, the NSView bridge is a contentView, at the root. These mimic
 // what TEST_VIEW usually does.
 TEST_F(BridgedNativeWidgetTest, BridgedNativeWidgetTest_TestViewAddRemove) {
   base::scoped_nsobject<BridgedContentView> view([bridge()->ns_view() retain]);
   EXPECT_NSEQ([test_window() contentView], view);
@@ skipping 453 matching lines @@
   [center postNotificationName:NSWindowDidExitFullScreenNotification
                         object:window];
   EXPECT_EQ(1, [window ignoredToggleFullScreenCount]);  // No change.
   EXPECT_FALSE(bridge()->target_fullscreen_state());
 
   widget_->CloseNow();
 }
 
 }  // namespace test
 }  // namespace views