cros: Allow --mus in about:flags on canary builds

Previously it was only allowed in developer builds and
dev channel builds. I think canary was an oversight.

Bug: none
Test: option appears in about:flags

Change-Id: I3611b0d23f43f53628e03bf1245ca056a1456c0f
Reviewed-on: https://chromium-review.googlesource.com/891693Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532569}

chrome/browser/about_flags.cc

@@ -3718,9 +3718,10 @@ class FlagsStateSingleton {
 bool SkipConditionalFeatureEntry(const FeatureEntry& entry) {
   version_info::Channel channel = chrome::GetChannel();
 #if defined(OS_CHROMEOS)
-  // Don't expose --mash/--mus outside of Canary or developer builds.
+  // Only expose --mash/--mus on unstable channels and developer builds.
   if (!strcmp("mus", entry.internal_name) &&
       channel != version_info::Channel::DEV &&
+      channel != version_info::Channel::CANARY &&
       channel != version_info::Channel::UNKNOWN) {
     return true;
   }