Tidy up JS_Defines.h

This is a purely mechanical change, no new functionality.

- Expand some macros which were merely a short-cut to save
  typing but reduced transparency.

- Put GET_VALUE_TYPE() implementation into a .cpp file.

This is a portion of the patch from issue 908033002 at
patchset 40001 (http://crrev.com/908033002#ps40001)

R=brucedawson@chromium.org

Review URL: https://codereview.chromium.org/927263003