Changes to ALLOW_UNUSED to match upcoming changes to the Chromium trunk:
* Eliminate usage of ALLOW_UNUSED to define COMPILE_ASSERT and just use
  static_assert() in all cases now that all platforms build with C++11.
* Convert remaining uses of ALLOW_UNUSED to ALLOW_UNUSED_TYPE to match how
  Chromium will be splitting this functionality.  (In Chromium we'll have both
  ALLOW_UNUSED_TYPE and ALLOW_UNUSED_LOCAL, which have different syntax to
  enable us to use these with MSVC.)

BUG=chromium:81439
TEST=none
LOG=y
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/613143004

Patch from Peter Kasting <pkasting@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24344 ce2b1a6d-e550-0410-aec6-3dcde31c8c00