Fix the native event created from WebKeyboardEvent.

Apparently some MacOS platform code (e.g processing
shortcuts for menu items in NSMenu) uses this hidden
field eventRef in NSEvent which is initalized on first
access. Apparently they access it in a way that it
doesn't get initialized. So we need to initialize
it in our code.

Bug: 881827
Change-Id: I6192b9cb8da3e5bc3fe20630f338cfd873efe877
Reviewed-on: https://chromium-review.googlesource.com/c/1351597Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: Navid Zolghadr <nzolghadr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611718}

content/browser/renderer_host/native_web_keyboard_event_mac.mm

@@ -84,6 +84,12 @@ NativeWebKeyboardEvent::NativeWebKeyboardEvent(
             charactersIgnoringModifiers:unmodified_text
                               isARepeat:NO
                                 keyCode:web_event.native_key_code] retain];
+  // The eventRef is necessary for MacOS code (like NSMenu) to work later in the
+  // pipeline. As per documentation:
+  // https://developer.apple.com/documentation/appkit/nsevent/1525143-eventref
+  // "Other NSEvent objects create an EventRef when this property is first
+  // accessed, if possible".
+  [os_event eventRef];
 }
 
 NativeWebKeyboardEvent::NativeWebKeyboardEvent(gfx::NativeEvent native_event)