Set default Managed Display mode when display zoom is used

ARC++ gets its device scale factor from the default ManagedDisplayMode.
When display zoom is enabled, we only have 1 display mode for the
internal display. This display mode was not set as the default until
now, causing ARC++ to get an invalid device scale factor.

Bug: 790723
Change-Id: Ia86a3c22edb21ea3d0ced63f31992cca3e700ad6
Component: Managed Display Mode, Device scale factor, display zoom
Reviewed-on: https://chromium-review.googlesource.com/964795Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Malay Keshav <malaykeshav@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543554}

ui/display/manager/chromeos/display_change_observer.cc

@@ -68,8 +68,10 @@ DisplayChangeObserver::GetInternalManagedDisplayModeList(
                                  display_info.device_scale_factor());
   // When display zoom option is available, we cannot change the mode for
   // internal displays.
-  if (chromeos::switches::IsDisplayZoomSettingEnabled())
+  if (chromeos::switches::IsDisplayZoomSettingEnabled()) {
+    native_mode.set_is_default(true);
     return ManagedDisplayInfo::ManagedDisplayModeList{native_mode};
+  }
   return CreateInternalManagedDisplayModeList(native_mode);
 }