Encoding API: Add ignoreBOM flag to decoder options

An additional option flag was added to the WHATWG Encoding Living
Standard's API to allow callers to disable the minimal BOM handling
the API supports.

BUG=362609

Review URL: https://codereview.chromium.org/232513003

git-svn-id: svn://svn.chromium.org/blink/trunk@174918 bbb929c8-8fbe-4397-9dbb-9b2b20218538