Revert r174506

r174506 replaced toV8Context() with v8::Object::CreationContext(),
and it caused 16% regression in dromaeo.jslibeventjquery.
This is because v8::Object::CreationContext() is slow.

This CL reverts the change.

BUG=383926

Review URL: https://codereview.chromium.org/429433002

git-svn-id: svn://svn.chromium.org/blink/trunk@179085 bbb929c8-8fbe-4397-9dbb-9b2b20218538