The JSNativeContextSpecialization class is getting rather huge with all
the stuff related to property and element access going in. Splitting off
the global object related stuff into JSGlobalObjectSpecialization seems
like a natural separation, especially since the global object
specialization is sort of separate issue anyway.  This is neutral
functionality- and performance-wise.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1417043006

Cr-Commit-Position: refs/heads/master@{#31748}