Bug 1259916: [MSE] P1. Fix eviction. r=gerald

Bug 1216460 introduced a regression which would cause to always evict from both ends of the current track buffer.

MozReview-Commit-ID: 4f01tNQ2KU8

--HG--
extra : rebase_source : 7da96fc1cbca03d60705fce681ee9259dd81d173

dom/media/mediasource/TrackBuffersManager.cpp

@@ -475,6 +475,8 @@ TrackBuffersManager::DoEvictData(const TimeUnit& aPlaybackTime,
     partialEvict += frame->ComputedSizeOfIncludingThis();
   }
 
+  const int64_t finalSize = mSizeSourceBuffer - aSizeToEvict;
+
   if (lastKeyFrameIndex > 0) {
     MSE_DEBUG("Step1. Evicting %lld bytes prior currentTime",
               aSizeToEvict - toEvict);
@@ -483,8 +485,7 @@ TrackBuffersManager::DoEvictData(const TimeUnit& aPlaybackTime,
                    TimeUnit::FromMicroseconds(buffer[lastKeyFrameIndex]->mTime - 1)));
   }
 
-  const int64_t finalSize = mSizeSourceBuffer - aSizeToEvict;
-  if (mSizeSourceBuffer <= finalSize || !buffer.Length()) {
+  if (mSizeSourceBuffer <= finalSize) {
     return;
   }