Initial implementation of new-style notification

This works behind flag. Currently only normal notification is supported.

BUG=723144

Review-Url: https://codereview.chromium.org/2892893002
Cr-Commit-Position: refs/heads/master@{#475798}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index dd8a2e27..026af141 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1851,6 +1851,13 @@
      kOsDesktop,
      SINGLE_VALUE_TYPE(
          switches::kEnableMessageCenterAlwaysScrollUpUponNotificationRemoval)},
+    {"enable-message-center-new-style-notification",
+     flag_descriptions::kMessageCenterNewStyleNotificationName,
+     flag_descriptions::kMessageCenterNewStyleNotificationDescription,
+     kOsDesktop,
+     ENABLE_DISABLE_VALUE_TYPE(
+         switches::kEnableMessageCenterNewStyleNotification,
+         switches::kDisableMessageCenterNewStyleNotification)},
 #endif  // !OS_ANDROID
     {"enable-md-policy-page",
      flag_descriptions::kEnableMaterialDesignPolicyPageName,