Modify BMP alpha decoding to be more similar to other browsers.

This results in ignoring the alpha channel data more often.

BUG=38618
TEST=none
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/271553002

git-svn-id: svn://svn.chromium.org/blink/trunk@173548 bbb929c8-8fbe-4397-9dbb-9b2b20218538