Added fallback to createGainNode method call for webkit implementation when creating global mute node
Added fallback to createGainNode method call for webkit implementation when creating global mute node

rblopes/phaser,BeanSeed/phaser,spayton/phaser,photonstorm/phaser,pixelpicosean/phaser,TukekeSoft/phaser,photonstorm/phaser,englercj/phaser,TukekeSoft/phaser,mahill/phaser,mahill/phaser,BeanSeed/phaser,spayton/phaser,TukekeSoft/phaser