Ignore unsupported disposal methods when decoding GIF

Avoid assertion failures about invalid disposal methods by ignoring
unsupported values when decoding GIF.

The bug is caused by some GIF images containing frames with disposal
method 5. Don't know what the author wanted.

BUG=302034
R=aelias@chromium.org, pkasting@chromium.org

Review URL: https://codereview.chromium.org/25422003

git-svn-id: svn://svn.chromium.org/blink/trunk@158769 bbb929c8-8fbe-4397-9dbb-9b2b20218538