Remove unnecessary attribute "gain" in AudioBuffer and AudioBufferSourceNode interfaces 

It has been removed in recent specification which is not used in AudioBuffer and AudioBufferSourceNode. 
There was nothing with "gain" attribute. 

http://www.w3.org/TR/webaudio/#AudioBuffer 
http://www.w3.org/TR/webaudio/#AudioBufferSourceNode-section

BUG=350928, 350770

Review URL: https://codereview.chromium.org/190973010

git-svn-id: svn://svn.chromium.org/blink/trunk@171073 bbb929c8-8fbe-4397-9dbb-9b2b20218538