Add a new flag and change system bubble background

This CL
* Adds a new flag --enable-ash-new-system-menu.
  We already have --enable-ash-sidebar flag, but we are going to do
  system menu restyling and system menu & notification center
  integration separately.

  Alternative considered: Instead of adding a new flag, we considered
  using FeatureVariation in about_flags.cc, but it seems no one in ash/
  is using this, and the primary use case of FeatureVariation looks
  different from this.

* Changes background and icon colors of system menu.
  These views are still subject to change, or will be even removed,
  as they are still largely different from the spec.

Spec: http://shortn/_1MHrIdbuHM
Design doc: go/cros-qs-restyling

TEST=manual
BUG=811641

Change-Id: I8da914e02ebfe67519f07ea3b9f3ee16c96ccbff
Reviewed-on: https://chromium-review.googlesource.com/917842
Commit-Queue: Tetsui Ohkubo <[email protected]>
Reviewed-by: James Cook <[email protected]>
Cr-Commit-Position: refs/heads/master@{#537185}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 0790942..4106f249 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -132,6 +132,7 @@
 #endif  // OS_ANDROID
 
 #if defined(OS_CHROMEOS)
+#include "ash/public/cpp/ash_features.h"
 #include "ash/public/cpp/ash_switches.h"
 #include "chromeos/chromeos_switches.h"
 #include "components/arc/arc_features.h"
@@ -1970,6 +1971,9 @@
      flag_descriptions::kAshSidebarDescription, kOsCrOS,
      ENABLE_DISABLE_VALUE_TYPE(ash::switches::kAshSidebarEnabled,
                                ash::switches::kAshSidebarDisabled)},
+    {"enable-ash-new-system-menu", flag_descriptions::kAshNewSystemMenuName,
+     flag_descriptions::kAshNewSystemMenuDescription, kOsCrOS,
+     FEATURE_VALUE_TYPE(ash::features::kAshNewSystemMenu)},
 #endif  // OS_CHROMEOS
     {"enable-message-center-new-style-notification",
      flag_descriptions::kMessageCenterNewStyleNotificationName,