Modify ALLOW_UNUSED to make it effective on MSVC as well.

This requires a syntax change.  After feedback from various people, it seems
like perhaps the least offensive transformation is

int x ALLOW_UNUSED = 0;

-->

int x = 0;
ALLOW_UNUSED_LOCAL(x);

The name changed for a couple of reasons:
(1) Various files #include both the wtf headers and the Chromium base headers
which both define this macro.  To avoid compile breaks when changing one
definition before the other, either both places have to gain #ifndef guards, or
the names have to (at least temporarily) be changed to not match.
(2) On the Chromium side, the macro is going to be split into ALLOW_UNUSED_LOCAL
and ALLOW_UNUSED_TYPE, with the latter used for e.g. possibly-unused typedefs.
These must be annotated on gcc but not on MSVC, and can't use the same
implementation as ALLOW_UNUSED_LOCAL() anyway, so ALLOW_UNUSED_TYPE will retain
the attribute-based implementation.  This is a long-winded way of saying that
the name change here is chosen to match an eventual name change there.

BUG=81439
TEST=none

Review URL: https://codereview.chromium.org/605423002

git-svn-id: svn://svn.chromium.org/blink/trunk@183159 bbb929c8-8fbe-4397-9dbb-9b2b20218538