Colored Emoji Font Support

This CL adds Emoji font support, which is needed for Chrome. Changes taken from 
https://chromium.googlesource.com/external/ots.git/+/6462c58fa123bb89baf1ec4b7538d8f2c682dc8b
This is done as per discussion with Behdad.
Thank you for quick review, will check suggested changes.
 Initial changes were shared just for quick review as suggested by Behdad as at that time my setup was not ready.
 I have submitted one more change with compile issue fix.
 Will check suggested changes and then share final patch.