Bug 1569706 - Update incorrect assertion. r=emilio

We can end up in here from a style worker thread.  But that's safe since
we only read data from the FontFaceSet, and the main thread is blocked
so writes can't happen underneath us.

Differential Revision: https://phabricator.services.mozilla.com/D44589

--HG--
extra : moz-landing-system : lando

layout/style/FontFaceSet.cpp

@@ -1204,9 +1204,8 @@ RawServoFontFaceRule* FontFaceSet::FindRuleForUserFontEntry(
 nsresult FontFaceSet::LogMessage(gfxUserFontEntry* aUserFontEntry,
                                  const char* aMessage, uint32_t aFlags,
                                  nsresult aStatus) {
-  MOZ_ASSERT(NS_IsMainThread(),
+  MOZ_ASSERT(NS_IsMainThread() ||
-             "LogMessage only works on the main thread, due to the Servo_XXX "
+             ServoStyleSet::IsCurrentThreadInServoTraversal());
-             "CSSOM calls it makes");
 
   nsCOMPtr<nsIConsoleService> console(
       do_GetService(NS_CONSOLESERVICE_CONTRACTID));