Raise kViEMaxNumberOfChannels from 32 to 64

Recent testing has shown that on modern desktops and laptops, decoding more than
32 low-resolution realtime video streams simultaneously is both possible and
desirable.

Reviewed:
https://webrtc-codereview.appspot.com/16449004/

TBR=mflodman
BUG=

Review URL: https://webrtc-codereview.appspot.com/17429004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6087 4adac7df-926f-26a2-2b94-8c16560cd09d