Add navigator.maxTouchPoints

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

BUG=248918

Review URL: https://codereview.chromium.org/27750002

git-svn-id: svn://svn.chromium.org/blink/trunk@160153 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce DEFINE_AX_OBJECT_TYPE_CASTS to replace manual toFoo() for accessibility child class.

DEFINE_TYPE_CASTS can be used for toFoo() in accessibility child class. The DEFINE_TYPE_CASTS will generate
plenty more helper functions for accessibility class type cast.

BUG=309516

Review URL: https://codereview.chromium.org/31463002

git-svn-id: svn://svn.chromium.org/blink/trunk@160154 bbb929c8-8fbe-4397-9dbb-9b2b20218538