When creating gray from lcd, use luminance instead of coverage.

[bungeman, 2012-08-28 15:09:11]

See http://code.google.com/p/chromium/issues/detail?id=144958

Part of the above was already fixed with https://codereview.appspot.com/6484048/
but this seems to correct the rest of it.

Basically, this makes skia regular anti-alias from system regular anti-alias
stay the way they are, but skia regular anti-alias from system subpixel
anti-alias preserve the visual weight.

[reed1, 2012-08-28 15:56:14]

lgtm

[bungeman, 2012-08-28 17:01:58]

Committed revision 5318.