Add feature flag to enlarge player pool

REDMINE-20506

config/initializers/features.rb

@@ -2,5 +2,6 @@
   config.features.register('webp_images')
   config.features.register('highdef_video_encoding')
   config.features.register('force_fullhd_video_quality')
+  config.features.register('large_player_pool')
   config.features.register('selectable_themes')
 end

package/spec/frontend/media/MediaPool_spec.js

@@ -80,6 +80,27 @@ describe('MediaPool', function() {
       expect(reusedPlayer).toBe(player);
       expect(player.textTracks().length).toEqual(0);
     });
+
+    it('supports supplying player count via function', () => {
+      const pool = new MediaPool({playerCount: playerType => {
+        if (playerType === MediaType.AUDIO) {
+          return 2;
+        }
+        else {
+          return 5;
+        }
+      }});
+
+      const audioPlayers = new Set(Array(10).fill().map(() =>
+        pool.allocatePlayer({playerType: MediaType.AUDIO})
+      ));
+      const videoPlayers = new Set(Array(10).fill().map(() =>
+        pool.allocatePlayer({playerType: MediaType.VIDEO})
+      ));
+
+      expect(audioPlayers.size).toEqual(2);
+      expect(videoPlayers.size).toEqual(5);
+    });
   });
 
   describe('#unallocatePlayer', function() {

package/src/frontend/media/MediaPool.js

@@ -147,7 +147,11 @@ export class MediaPool {
     return player;
   }
   initializeMediaPool_(type, mediaElSeed){
-    while ( this.allPlayersForType(type).length<this.playerCount ) {
+    const playerCount = typeof this.playerCount === 'function' ?
+                        this.playerCount(type) :
+                        this.playerCount;
+
+    while (this.allPlayersForType(type).length < playerCount ) {
       this.createPlayer_(type, mediaElSeed.cloneNode(true));
     }
   }

package/src/frontend/media/media.js

@@ -1,8 +1,11 @@
 import {MediaPool, MediaType} from './MediaPool';
 import BackboneEvents from 'backbone-events-standalone';
+import {features} from '../features';
 
 export const media = {
-  playerPool: new MediaPool(),
+  playerPool: new MediaPool({
+    playerCount: () => features.isEnabled('large_player_pool') ? 10 : 4
+  }),
   muteState: true,
   get muted(){ return this.muteState; },
   mute: function (value) {