Chromium side of maxTouchPoints implementation.

Intent to implement-and-ship: https://groups.google.com/a/chromium.org/forum/#!searchin/blink-dev/maxTouchPoints/blink-dev/ayzxdztUlOQ/rd-z_Jo3ocIJ

navigator.maxTouchPoints is defined in the W3C Pointer Events
standard draft:
http://www.w3.org/TR/pointerevents/#widl-Navigator-maxTouchPoints

Depends on blink revision 160153:
https://src.chromium.org/viewvc/blink?view=revision&revision=160153

BUG=248918

Review URL: https://codereview.chromium.org/26764002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231203 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231203