resourceloader: Avoid needless GPC normalisation

resourceloader: Avoid needless GPC normalisation

Utility methods like getFuzzyBool already use getRawVal instead
of getVal (see 83df6d8). But there was still direct use of getVal
which meant we still pay for WebRequest normalisation in load.php.

ResourceLoaderContext::__construct's use of WebRequest::getVal
accounts for 0.12% of execution time according to a Xenon daily log
(2016-09-27.all.reversed.svgz). The total use of WebRequest::getVal
is 0.26% of execution time. So almost half of that comes from RL.

Change-Id: Id37619ceadb5c990abf83f7910104cbc95523d28