Improve GIF image decoding by using a RGBA lookup table

Instead of computing RGBA value for every pixel precompute the color map
with RGBA values.

BUG=233076

Review URL: https://chromiumcodereview.appspot.com/20723010

git-svn-id: svn://svn.chromium.org/blink/trunk@155385 bbb929c8-8fbe-4397-9dbb-9b2b20218538