Let use_accelerated_compositing be usable for both enabling and disabling compositing

Let use_accelerated_compositing be usable for both enabling and disabling compositing.

With this, Chromium can be built on 10.5 again with the following ~/.gyp/include.gypi:


{
  'variables': {
    'build_ffmpegsumo': 0, 
    'use_system_ffmpeg': 0, 
    'disable_nacl': 1,
    'enable_svg': 0,
    'use_accelerated_compositing': 0,
  }
}

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=56871

[Nico, 2010-08-20 06:43:48]

This doesn't change the default, but now use_accelerated_compositing=0 turns the
feature off, which brings the binary size down enough to let chrome link again
on 10.5.

[Ken Russell (switch to Gerrit), 2010-08-20 07:05:53]

LGTM -- it seems a little weird that forcibly setting
use_accelerated_compositing=1 won't cause the code to be built on a new
platform, but at this point this is a reasonable default and change.