Bug 1627075 - Route Omnijar requests through StartupCache r=froydnj

This should be a relatively straightforward patch. Essentially, we implement
a wrapper class (and friends) around nsZipArchive (and friends), which transparently
caches entries from the underlying zip archive in the StartupCache. This will break
without changes to the StartupCache, made in the patch after this, which allow it
to be used off of the main thread, and outside the main process.

Depends on D77635

Differential Revision: https://phabricator.services.mozilla.com/D77634

js/xpconnect/loader/mozJSSubScriptLoader.cpp

@@ -24,6 +24,7 @@
 
 #include "mozilla/ContentPrincipal.h"
 #include "mozilla/dom/ScriptLoader.h"
+#include "mozilla/Omnijar.h"
 #include "mozilla/ScriptPreloader.h"
 #include "mozilla/SystemPrincipal.h"
 #include "mozilla/scache/StartupCache.h"
@@ -248,6 +249,10 @@ bool mozJSSubScriptLoader::ReadScript(JS::MutableHandle<JSScript*> script,
                                       const char* uriStr, nsIIOService* serv,
                                       bool wantReturnValue,
                                       bool useCompilationScope) {
+  // We're going to cache the XDR encoded script data - suspend writes via the
+  // CacheAwareZipReader, otherwise we'll end up redundantly caching scripts.
+  AutoSuspendStartupCacheWrites suspendScache;
+
   // We create a channel and call SetContentType, to avoid expensive MIME type
   // lookups (bug 632490).
   nsCOMPtr<nsIChannel> chan;