Refactor v8_types.v8_value_to_local_cpp_value for clarity and to make Promises easier to implement

Noticed this cleanup, which will also make reuse easier.

Support for CL:
API functions returning Promises should not throw exceptions.
https://codereview.chromium.org/232563003/

R=haraken
BUG=359386

Review URL: https://codereview.chromium.org/231833005

git-svn-id: svn://svn.chromium.org/blink/trunk@171262 bbb929c8-8fbe-4397-9dbb-9b2b20218538