Revert "deprecate Context::New which returns Persistent"

This reverts r14573 because of test failures in no-snapshot mode in the
cctest/test-debug/ScriptCollectedEventContext test case.

TBR=dcarney@chromium.org,svenpanne@chromium.org
TEST=cctest/test-debug/ScriptCollectedEventContext

Review URL: https://codereview.chromium.org/15038002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14578 ce2b1a6d-e550-0410-aec6-3dcde31c8c00