Bug 1540748 - part0 : mark functions const. r=jya

There functions won't change any internal variables, so they should be marked as const.

Differential Revision: https://phabricator.services.mozilla.com/D29590

--HG--
extra : moz-landing-system : lando

dom/media/platforms/android/RemoteDataDecoder.h

@@ -49,14 +49,16 @@ class RemoteDataDecoder : public MediaDataDecoder,
   void ReturnDecodedData();
   void DrainComplete();
   void Error(const MediaResult& aError);
-  void AssertOnTaskQueue() { MOZ_ASSERT(mTaskQueue->IsCurrentThreadIn()); }
+  void AssertOnTaskQueue() const {
+    MOZ_ASSERT(mTaskQueue->IsCurrentThreadIn());
+  }
 
   enum class State { DRAINED, DRAINABLE, DRAINING, SHUTDOWN };
   void SetState(State aState) {
     AssertOnTaskQueue();
     mState = aState;
   }
-  State GetState() {
+  State GetState() const {
     AssertOnTaskQueue();
     return mState;
   }