fix: Avoid a race when clearing buffered content

This changes the order of operations in clearing buffered content in
StreamingEngine.  Now we clear the media states (synchronously) right
before we clear the actual buffered content (asynchronously).

This fixes a test failure in "StreamingEngine VOD can handle
unbuffered seeks" with Firefox on Windows.

Fixes b/162752568

Change-Id: I27f98fa0e14d4e9e1f1a24da4f5fda85c457bd1d

lib/media/streaming_engine.js

@@ -1752,6 +1752,8 @@ shaka.media.StreamingEngine = class {
     mediaState.waitingToFlushBuffer = false;
     mediaState.clearBufferSafeMargin = 0;
     mediaState.clearingBuffer = true;
+    mediaState.lastStream = null;
+    mediaState.lastSegmentReference = null;
 
     shaka.log.debug(logPrefix, 'clearing buffer');
 
@@ -1772,8 +1774,6 @@ shaka.media.StreamingEngine = class {
     this.destroyer_.ensureNotDestroyed();
 
     shaka.log.debug(logPrefix, 'cleared buffer');
-    mediaState.lastStream = null;
-    mediaState.lastSegmentReference = null;
     mediaState.clearingBuffer = false;
     mediaState.endOfStream = false;
     this.scheduleUpdate_(mediaState, 0);