Simplify WTF Functional.h classes using variadic templates

SSIA. Using of variadic templates drastically reduces the code size in the Functional.h (-830 lines of code). 

The client code does not require any modifications.

Review URL: https://codereview.chromium.org/610603002

git-svn-id: svn://svn.chromium.org/blink/trunk@182915 bbb929c8-8fbe-4397-9dbb-9b2b20218538