Add toV8Value and createV8Entry which take creation context and isolate to WebDOMFileSystem.

BUG=369451

Review URL: https://codereview.chromium.org/297383002

git-svn-id: svn://svn.chromium.org/blink/trunk@174864 bbb929c8-8fbe-4397-9dbb-9b2b20218538