Bug 1193245 - Use Atomic in the suspend count of the media resource. r=jwwang

In present codebase, the |mSuspendCount| would be accessed in main thread and state machine thread, and we use the lock mechanism to avoid the race condition.

However, we can use another more easier way to achieve it, that is using Atomic for the |mSuspendCount|.

--HG--
extra : transplant_source : %E7%80%93h%25%5D%24%CA%0C%19%1BDQ%60%08%BD%CB%9D%90%FC

dom/media/MediaResource.cpp

@@ -1114,7 +1114,6 @@ ChannelMediaResource::IsSuspendedByCache()
 bool
 ChannelMediaResource::IsSuspended()
 {
-  MutexAutoLock lock(mLock);
   return mSuspendCount > 0;
 }
 

dom/media/MediaResource.h

@@ -16,6 +16,7 @@
 #include "nsIInterfaceRequestor.h"
 #include "MediaCache.h"
 #include "MediaData.h"
+#include "mozilla/Atomics.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/TimeStamp.h"
 #include "nsThreadUtils.h"
@@ -778,7 +779,7 @@ protected:
   // A data received event for the decoder that has been dispatched but has
   // not yet been processed.
   nsRevocableEventPtr<nsRunnableMethod<ChannelMediaResource, void, false> > mDataReceivedEvent;
-  uint32_t           mSuspendCount;
+  Atomic<uint32_t>   mSuspendCount;
   // When this flag is set, if we get a network error we should silently
   // reopen the stream.
   bool               mReopenOnError;