Evaluate ARGB values in linear space instead of sRGB

A comparison between the old behavior and the behavior proposed in
this change can be found in the following document:

https://docs.google.com/a/google.com/document/d/1x8g5VcEZbuZd-RhSITc1VPpZ_dc0FlF1WbDscBYPQzM

Change-Id: I29d3fdfdd45cb64f32ad6a6f3f25fa3dafe1a6d0