Build oxide with C++11

We build the core library with Chromium defaults (no C++11), which means we don't get features such as "final" and "override". When recently rebasing Oxide on the latest Chromium, I hit several weird test failures and bugs which ended up being due to the prototypes of various virtual functions we implement changing.

If we were able to use "final" or "override", the compiler would have caught these errors for me.

Note, Chromium currently does not build successfully with C++11