Backed out 2 changesets (bug 1544037) for build bustages at StartupCache.cpp on a CLOSED TREE

Backed out changeset 8242c6c52f6c (bug 1544037)
Backed out changeset c19e86931d1a (bug 1544037)

browser/base/content/test/performance/browser_startup_mainthreadio.js

@@ -188,6 +188,12 @@ const startupPhases = {
       condition: WIN,
       stat: 1,
     },
+    { // bug 1544037
+      path: "ProfLDS:startupCache/startupCache." +
+             (Services.appinfo.is64Bit ? 8 : 4) + ".little",
+      condition: WIN,
+      stat: 1,
+    },
     { // bug 1541601
       path: "PrfDef:channel-prefs.js",
       stat: 1,

startupcache/StartupCache.cpp

@@ -204,13 +204,13 @@ nsresult StartupCache::Init() {
 nsresult StartupCache::LoadArchive() {
   if (gIgnoreDiskCache) return NS_ERROR_FAILURE;
 
+  bool exists;
+  mArchive = nullptr;
+  nsresult rv = mFile->Exists(&exists);
+  if (NS_FAILED(rv) || !exists) return NS_ERROR_FILE_NOT_FOUND;
+
   mArchive = new nsZipArchive();
   rv = mArchive->OpenArchive(mFile);
-
-  if (NS_FAILED(rv)) {
-    mArchive = nullptr;
-  }
-
   return rv;
 }