Reland^2 r22105 "Remove static initializer from isolate"

Reland^2 r22105 "Remove static initializer from isolate"

Make Isolate::UncheckedCurrent() return NULL if Isolate wasn't yet initialized
to avoid reentrancy when running with the profiler

BUG=none
R=dcarney@chromium.org
LOG=n

Committed: https://code.google.com/p/v8/source/detail?r=22167

[dcarney, 2014-07-02 09:34:32]

lgtm

[jochen (gone - plz use gerrit), 2014-07-02 14:44:02]

Committed patchset #2 manually as r22167 (presubmit successful).