Reviewed by Darin Adler.
Fixes: https://bugs.webkit.org/show_bug.cgi?id=22876
Unify random number generation in JavaScriptCore & WebCore, by introducing
wtf/RandomNumber.h and moving wtf_random/wtf_random_init out of MathExtras.h.
wtf_random_init() has been renamed to initializeRandomNumberGenerator() and
lives in it's own private header: wtf/RandomNumberSeed.h, only intended to
be used from within JavaScriptCore.
wtf_random() has been renamed to randomNumber() and lives in a public header
wtf/RandomNumber.h, usable from within JavaScriptCore & WebCore. It encapsulates
the code taking care of initializing the random number generator (only when
building without ENABLE(JSC_MULTIPLE_THREADS), otherwhise initializeThreading()
already took care of that).
Functional change on darwin: Use random() instead of rand(), as it got a larger
period (more randomness). HTMLFormElement already contains this implementation
and I just moved it in randomNumber(), as special case for PLATFORM(DARWIN).
