Change the BMP decoder to not reject paletted images which use color indexes
beyond the end of the color table.  Instead, draw these pixels as black.

This matches Firefox and IE.

BUG=176678
TEST=none
R=abarth@chromium.org, hclam@chromium.org

Review URL: https://codereview.chromium.org/213613002

git-svn-id: svn://svn.chromium.org/blink/trunk@170193 bbb929c8-8fbe-4397-9dbb-9b2b20218538