Add a bug link to mash browser test blacklist.

Bug: none
Change-Id: I7f3f331b0023077348049edbfb9ce104bee7a293
Reviewed-on: https://chromium-review.googlesource.com/1040306Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555522}

testing/buildbot/filters/mash.browser_tests.filter

@@ -51,6 +51,7 @@
 
 # ash::Shell access from ChromeViewsDelegate::CreateDefaultNonClientFrameView()
 # e.g. from chromeos::CaptivePortalWindowProxy::Show().
+# See https://crbug.com/838974
 -CaptivePortalWindowCtorDtorTest.*
 -CaptivePortalWindowTest.*
 -SimpleWebViewDialogTest.*