IDL compiler: rename value => jsValue, v => cppValue

Simple but tedious renaming, to clearly separate *which* value.
Also makes any future renamings easier (sed on 'v' is a bit fraught).

It's striking how much simpler the renaming is in Python,
(admittedly still only partially implemented)
since it's mostly changes in templates, not code.

BUG=239771

Review URL: https://codereview.chromium.org/26887006

git-svn-id: svn://svn.chromium.org/blink/trunk@159870 bbb929c8-8fbe-4397-9dbb-9b2b20218538