Move most of ChromeV8Context to a base ScriptContext
This also moves ModuleSystem, RequestSender,
ObjectBackedNativeHandler, and SafeBuiltins
over to //extensions.
Other renderer code is updated minimally to support
these changes.
BUG=359836
TBR=sky@chromium.org for chrome/test
TBR=jamescook@chromium.org for apps/shell
TBR=maruel@chromium.org for PRESUBMIT.py
Review URL: https://codereview.chromium.org/
234413005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263429 0039d316-1c4b-4281-b951-d872f2087c98