Fix ol.BrowserFeature.HAS_TOUCH detection code

Use goog.global instead goog.global.document.documentElement to avoid
false negative results when using the chrome development tools (Touch
Events emulation).

Code borrowed from https://github.com/Modernizr/Modernizr/blob/master/feature-detects/touchevents.js
except for the DocumentTouch part which is deprecated (https://developer.mozilla.org/en-US/docs/Web/API/DocumentTouch)

Fixes #1869
Merge pull request #1870 from fredj/has-touch

Fix ol.BrowserFeature.HAS_TOUCH detection code