macviews: never show mnemonics

Even when the parent MenuButton has keyboard focus, mnemonics should never
show on Mac.

Bug: 836964
Change-Id: If3906f664387c2730bc18f2c2bf638de6025c597
Reviewed-on: https://chromium-review.googlesource.com/1050068Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558300}

ui/views/controls/menu/menu_runner_impl.cc

@@ -204,6 +204,8 @@ bool MenuRunnerImpl::ShouldShowMnemonics(MenuButton* button) {
   show_mnemonics |= ui::win::IsAltPressed();
 #elif defined(USE_X11)
   show_mnemonics |= ui::IsAltPressed();
+#elif defined(OS_MACOSX)
+  show_mnemonics = false;
 #endif
   return show_mnemonics;
 }