Increase WebTouchEvent::touchesLengthCap to 16.

Pixel supports 16 touch-points but Blink caps the number at 12. This is change 2 of 3 on our way to update Blink's limit to 16.

BUG=392959

Review URL: https://codereview.chromium.org/637103002

git-svn-id: svn://svn.chromium.org/blink/trunk@183546 bbb929c8-8fbe-4397-9dbb-9b2b20218538