Simplify Gradient by holding a Color instead of four floats.

This makes it easier to read, and also prevents pointless conversions
back and forth between a 32-bit integer and four floats.

This resolves a FIXME which asked why this was the case.

BUG=

Review URL: https://codereview.chromium.org/201503002

git-svn-id: svn://svn.chromium.org/blink/trunk@169396 bbb929c8-8fbe-4397-9dbb-9b2b20218538