disable background hang monitor, and import changes from tenfourfox:

- #434: initial work on decode startup delay and MSE spoofing (1c7323d76)
- #431: M1377257 M1372602 (852497478)
- #417: remove more code from LookAndFeel; tune up a bit (079200623)
- #434: put this on hold for the moment (8a6fd1e65)
- #432: M1390342 (6aee6c06a)
- #334: remove telemetry from customizeableui (bbe74e8ef)
- #433: M1390550 libhyphen update (693b6a16e)
- #435: M1395157 (19a216628)
- #334: remove telemetry from xpcom (71b8c3d25)
- #433: M1390980 M1376306 M1390550 M1376825 M1380824 M1385272 (94d5cb15d)
- #437, #417: tune up LookAndFeel; some more 10.7+ removals (981c2e8fe)
- #437: whoops, forgot something (127c2fca4)
- #433: update certs and pins (023db21f4)
- tune up debug a bit (150b92b0f)
- #433: M1380292 M1371657 M1393624 M1386905 (e805f4188)
- whoops pick up one more libhyphen file (462625006)
- #433: M1395598 M1389974 M1396570 M1384801 (e13d43ac6)
- #433: M1368269 M1400399 (ea7b13db8)
- #433: update certs (b10941349)
- #72: let's try looking at this a different way (debug only) (9ba7a9029)

dom/storage/DOMStorageIPC.cpp

@@ -443,6 +443,7 @@ public:
   virtual bool LoadItem(const nsAString& aKey, const nsString& aValue)
   {
     // Called on the aCache background thread
+    MOZ_ASSERT(!mLoaded);
     if (mLoaded) {
       return false;
     }
@@ -457,8 +458,12 @@ public:
   {
     // Called on the aCache background thread
     MonitorAutoLock monitor(mMonitor);
+    MOZ_ASSERT(!mLoaded && mRv);
     mLoaded = true;
-    *mRv = aRv;
+    if (mRv) {
+      *mRv = aRv;
+      mRv = nullptr;
+    }
     monitor.Notify();
   }