fix(offline): Fix error when un-storing DRM asset

The DRM engine, when asked to remove a session, would then
remove it again when destroyed.
This changes the DRM engine to no longer track removed sessions
as active.

Closes shaka-project#3534

Change-Id: I2a3b99eb97e1d8f68282062c592a7cd35ffbe3cd

lib/media/drm_engine.js

@@ -550,6 +550,7 @@ shaka.media.DrmEngine = class {
     tasks.push(session.remove());
 
     await Promise.all(tasks);
+    this.activeSessions_.delete(session);
   }
 
   /**

test/media/drm_engine_unit.js

@@ -2285,6 +2285,19 @@ describe('DrmEngine', () => {
       await expectAsync(drmEngine.removeSession('abc'))
           .toBeRejectedWith(expected);
     });
+
+    // Regression test for #3534
+    it('does not remove the same session again on destroy', async () => {
+      updatePromise.resolve();
+      expect(session1.remove).not.toHaveBeenCalled();
+      await drmEngine.removeSession('abc');
+      expect(session1.remove).toHaveBeenCalled();
+      session1.remove.calls.reset();
+      await drmEngine.destroy();
+      // The session should only be removed ONCE. If it's double-removed, it
+      // will make a (non-fatal) DOMException.
+      expect(session1.remove).not.toHaveBeenCalled();
+    });
   });
 
   describe('expiration', () => {