Enable shelf dimming in guest mode.

Bug: 1049281
Change-Id: Ib9d4ada81de8568cbfc5f7dc7f69ef3eb6470daa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042192
Auto-Submit: Yulun Wu <yulunwu@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739107}

chrome/browser/chromeos/login/chrome_restart_request.cc

@@ -166,6 +166,7 @@ void DeriveCommandLine(const GURL& start_url,
     ash::switches::kAshEnablePaletteOnAllDisplays,
     ash::switches::kAshTouchHud,
     ash::switches::kAuraLegacyPowerButton,
+    ash::switches::kEnableDimShelf,
     ash::switches::kShowTaps,
     ash::switches::kShowWebUiLock,
     ash::switches::kShowWebUiLogin,