When using C++11, use static_assert to implement COMPILE_ASSERT.

Tag COMPILE_ASSERT with ALLOW_UNUSED for non-C++11 GCC 4.8, which otherwise
produces warnings under -Wunused-local-typedefs. That warning is currently
disabled in Chromium (r194779), but the ultimate goal should be to enable that
warning.

BUG=321833
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/77343005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236356 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236356