Add null check in -[NativeWidgetMacNSWindow saveRestorableState]

TBR=sdy

Bug: 1013250
Change-Id: I9090602c6af0d01b971b6a9764e21d9c1e50987f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864319Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707091}

components/remote_cocoa/app_shim/native_widget_mac_nswindow.mm

@@ -319,6 +319,8 @@
 }
 
 - (void)saveRestorableState {
+  if (!bridge_)
+    return;
   if (![self _isConsideredOpenForPersistentState])
     return;
   base::scoped_nsobject<NSMutableData> restorableStateData(