Client Hints

Client Hints can be used as input to proactive content negotiation;
just as the Accept header allowed clients to indicate what formats
they prefer, Client Hints allow clients to indicate a list of device
and agent specific preferences.

See:
https://github.com/igrigorik/http-client-hints/blob/draft2/draft-grigorik-http-client-hints-01.txt

Patched from https://codereview.chromium.org/23654014/, which was
patched from https://codereview.chromium.org/11970002

BUG=170388
CONTRIBUTOR=yoav@yoav.ws

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=225425

Review URL: https://codereview.chromium.org/24451003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227178 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227178
cc: Don't always push properties for scrollbar layers

R=danakj@chromium.org
BUG=259095

Review URL: https://codereview.chromium.org/25353005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227179 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227179