Bug 1532253 - Hold lock in mozilla::dom::IsType() r=baku

Differential Revision: https://phabricator.services.mozilla.com/D24018

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

dom/file/uri/BlobURLProtocolHandler.cpp

@@ -956,8 +956,8 @@ namespace mozilla {
 namespace dom {
 
 bool IsType(nsIURI* aUri, DataInfo::ObjectType aType) {
-  MOZ_ASSERT(NS_IsMainThread(),
-             "without locking gDataTable is main-thread only");
+  // We lock because this may be called off-main-thread
+  StaticMutexAutoLock lock(sMutex);
   DataInfo* info = GetDataInfoFromURI(aUri);
   if (!info) {
     return false;