Clang seems to support C99-but-not-C++ features as an extension.  An example is designated initializer lists, which are still on the drawing board for C++ (slated for C++20, and with less power/flexibility than C99).

IMO, we should not support these sorts of things in Chrome.  The C++ standards committee thinks about the question of how to maintain compatibility with C over time, so until they allow something into C++ itself, I would defer to their judgment.

Apparently a -Wc99-extensions flag exists to warn on such things.  We should try turning it on, seeing how bad things are, and if possible fixing the errors and leaving it enabled.