Refactor V8StringResource::prepare

V8StringResource<T>::prepareBase was not a good name.
Because V8StringResource<T>::prepare added some additional checks depending
on T, I renamed them to "check", added the check to prepareBase and rename it
to "prepare". This is a refactoring CL and doesn't change the behavior.

BUG=NONE

Review URL: https://codereview.chromium.org/276613007

git-svn-id: svn://svn.chromium.org/blink/trunk@173717 bbb929c8-8fbe-4397-9dbb-9b2b20218538