Bug 816656 - Don't write the startup cache on shutdown. r=taras

The startup cache will be saved in the background ~10 seconds after startup. If 10 seconds haven't elapsed yet don't bother write the cache on shutdown.

startupcache/StartupCache.cpp

@@ -139,7 +139,9 @@ StartupCache::~StartupCache()
   // but an early shutdown means either mTimer didn't run 
   // or the write thread is still running.
   WaitOnWriteThread();
-  WriteToDisk();
+  // If we shutdown quickly timer wont have fired. Instead of writing
+  // it on the main thread and block the shutdown we simply wont update
+  // the startup cache.
   gStartupCache = nullptr;
   (void)::NS_UnregisterMemoryReporter(mMappingMemoryReporter);
   (void)::NS_UnregisterMemoryReporter(mDataMemoryReporter);