Remove extern "C" from Assertions.h, it is no longer useful.

It is only included in C++ sources.

BUG=None

Review URL: https://codereview.chromium.org/583323002

git-svn-id: svn://svn.chromium.org/blink/trunk@182410 bbb929c8-8fbe-4397-9dbb-9b2b20218538