Added workaround for window.scrollX compat.

window.scrollX/Y is not available in IE11. As far as specifications go, is currently only specified in draft (http://dev.w3.org/csswg/cssom-view/#refsCSSOM). Falling back to 
window.pageXOffset seems like a good workaround.

On a related note; my Emscriptified project runs on IE11 although performance is very poor (mostly due to Internet Explorer itself, I think). It's pretty finicky about the shaders, as they introduced an extra set of requirements. (inout/in/out keywords not supported, can't construct mat3 from mat4, etc).
Added comments & assert to scrollX fix.

Added comments to fix as per @kripken's request.
Added an assert too (guarded by ASSERTIONS define)
Fixed undefined check.

Forgot I had to explicitly check the type of variable type to see if it's undefined.
Update library_browser.js
Merge pull request #1941 from onnoj/incoming

Added workaround for window.scrollX compat.