Bug 1351200 - Part 1: stylo: Allow fetching prefs on servo traversal threads; r=froydnj

MozReview-Commit-ID: 7MMWyX1t5CR

--HG--
extra : rebase_source : c773ed9157d65934345aa02adf2242055cb41e64

modules/libpref/prefapi.cpp

@@ -26,6 +26,7 @@
 #include "mozilla/ArenaAllocatorExtensions.h"
 #include "mozilla/Logging.h"
 #include "mozilla/MemoryReporting.h"
+#include "mozilla/ServoStyleSet.h"
 #include "mozilla/dom/PContent.h"
 #include "mozilla/dom/ContentPrefs.h"
 #include "nsQuickSort.h"
@@ -762,7 +763,7 @@ inInitArray(const char* key)
 PrefHashEntry* pref_HashTableLookup(const char *key)
 {
 #ifndef MOZ_B2G
-    MOZ_ASSERT(NS_IsMainThread());
+    MOZ_ASSERT(NS_IsMainThread() || mozilla::ServoStyleSet::IsInServoTraversal());
 #endif
     MOZ_ASSERT((!XRE_IsContentProcess() || gPhase != START),
                "pref access before commandline prefs set");