Bug 1323847: [MSE] P4. Bump audio buffer size. r=gerald

Under some cases YouTube attempts to append more than 10MB of data ahead of currentTime. This causes the appendBuffer to be rejected with QUOTA_EXCEEDED_ERR as as per spec.

Bug 1320829 slightly increased the size of the MediaRawData object (by 36 bytes) which on average caused an increase of 470kB of the source buffer size causing the eviction threshold to be crossed quicker.
YouTube clears the entire source buffer once a buffer full is reported and reloads it all, causing an audible silence.

Bumping the threshold slightly is the only way to get around the problem.

MozReview-Commit-ID: HgtHFcZHUG1

--HG--
extra : rebase_source : 74b78a551c5eb827576d1797928cc4da51eb3dd1

dom/media/mediasource/TrackBuffersManager.cpp

@@ -100,7 +100,7 @@ TrackBuffersManager::TrackBuffersManager(MediaSourceDecoder* aParentDecoder,
   , mVideoEvictionThreshold(Preferences::GetUint("media.mediasource.eviction_threshold.video",
                                                  100 * 1024 * 1024))
   , mAudioEvictionThreshold(Preferences::GetUint("media.mediasource.eviction_threshold.audio",
-                                                 10 * 1024 * 1024))
+                                                 12 * 1024 * 1024))
   , mEvictionState(EvictionState::NO_EVICTION_NEEDED)
   , mMonitor("TrackBuffersManager")
 {