Properly decode BMP color values with components < 8 bits in size.

BUG=360828
TEST=See bug
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/259753008

git-svn-id: svn://svn.chromium.org/blink/trunk@172802 bbb929c8-8fbe-4397-9dbb-9b2b20218538