Bug 1595596 - Always wait on Prefetch thread before touching mCacheData r=froydnj

This is not, AFAICT, causing crashes - it is just an oversight noticed while
investigating the crash in the bug.

Differential Revision: https://phabricator.services.mozilla.com/D53042

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

startupcache/StartupCache.cpp

@@ -276,6 +276,7 @@ Result<Ok, nsresult> StartupCache::LoadArchive() {
       return Err(NS_ERROR_UNEXPECTED);
     }
     auto cleanup = MakeScopeExit([&]() {
+      WaitOnPrefetchThread();
       mTable.clear();
       mCacheData.reset();
     });
@@ -564,6 +565,7 @@ Result<Ok, nsresult> StartupCache::WriteToDisk() {
 
 void StartupCache::InvalidateCache(bool memoryOnly) {
   WaitOnWriteThread();
+  WaitOnPrefetchThread();
   mWrittenOnce = false;
   if (memoryOnly) {
     auto writeResult = WriteToDisk();