resourceloader: Remove redundant 'window' indirection

resourceloader: Remove redundant 'window' indirection

Use global variables directly as globals, except for the specific
case of accessing it when it may not exist. In those cases we
use the fact that undefined properties of an object yield the
`undefined` value which we can cast to false. Accessing an undefined
variable would yield a ReferenceError exception.

Change-Id: I1d9e9aa5845ba3c756ad6e31358d8594e003b04b