Make sure we register extensions only once. Removed unused member variable.

Make sure we register extensions only once. Removed unused member variable.

V8::Initialize() can be called from various threads simultaneously, so
everything should be done only once per process or within the Isolate. For
registering extensions, we do the former now.

R=dslomov@chromium.org

Committed: http://code.google.com/p/v8/source/detail?r=14730

[Dmitry Lomov (no reviews), 2013-05-21 11:59:37]

lgtm

[Sven Panne, 2013-05-21 12:03:57]

Committed patchset #1 manually as r14730 (presubmit successful).