This is essentially a port of http://crrev.com/2403003002 to TurboFan,
adding support for fast access to JSGlobalObject properties through the
current native contexts' JSGlobalProxy.

It's a slightly bigger change, since JSNativeContextSpecialization and
JSGlobalObjectSpecialization needs merging for this to work, as due to
different type feedback layout we cannot just turn a JSLoadNamed into
JSLoadGlobal operator (and same for JSStoreNamed vs. JSStoreGlobal).
This part of the change is mostly mechanical.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng
R=ishell@chromium.org, jochen@chromium.org
BUG=chromium:634276,v8:5267

Review-Url: https://codereview.chromium.org/2664853002
Cr-Commit-Position: refs/heads/master@{#42805}