Replace JS_THIS_OBJECT and JS::CallReceiver in preparation for SpiderMonkey 49 and 61.
Differential Revision: https://code.wildfiregames.com/D2173
Refs #4893, D1699, D2142, D844, rP7259, ...
Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1270977
Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1255800
Tested on: clang 8, VS2015, Jenkins