Minor improvements to Context creation

Minor improvements to Context creation

Avoid converting the this object for builtins, and
remove the GC blocker that's not needed anymore.
Move some parts of the code out to the caller
of initCallContext().

Change-Id: I8b11dd1604fb14940e4d4e9f403c2a8bfc837c4b
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>