Merge G1ConcurrentMark::par_mark() and G1ConcurrentMark::grayRoot()

The methods G1ConcurrentMark::par_mark() and G1ConcurrentMark::grayRoot() both mark bits in the next mark bitmap. They or their callers always check whether that mark should actually be set before doing it in various different ways. I.e. somewhat obfuscated code duplication.

Remove that code duplication.