Align SkLumaColorFilter with the spec.

Align SkLumaColorFilter with the spec.

The spec requires the resulting RGB channels for LuminanceToAlpha to be
0 (and not just scaled by the luminance value</facepalm>):

  | R' |     |      0        0        0  0  0 |   | R |
  | G' |     |      0        0        0  0  0 |   | G |
  | B' |  =  |      0        0        0  0  0 | * | B |
  | A' |     | 0.2125   0.7154   0.0721  0  0 |   | A |
  | 1  |     |      0        0        0  0  1 |   | 1 |

(http://www.w3.org/TR/2011/REC-SVG11-20110816/filters.html#feColorMatrixElement)

This doesn't affect luminance masking (which depends only on the
resulting alpha channel), but other color filter users may care
about all color components.

R=bsalomon@google.com, reed@google.com, robertphillips@google.com

Author: fmalita@chromium.org

Review URL: https://codereview.chromium.org/26467003

git-svn-id: http://skia.googlecode.com/svn/trunk@11713 2bbb7eff-a529-9590-31e7-b0007b416f81