Introduce ToV8Value.

ToV8Value is a utility class for converting an object to a V8 value.
This is a refactoring CL.

BUG=NONE
R=haraken

Review URL: https://codereview.chromium.org/246463002

git-svn-id: svn://svn.chromium.org/blink/trunk@172118 bbb929c8-8fbe-4397-9dbb-9b2b20218538