[CSS Grid] Introduce an explicit type for resolved grid positions

[CSS Grid] Introduce an explicit type for resolved grid positions

We were using simple size_t integers to represent resolved grid
positions in our internal data structures. This change allows us to
clarify the code, avoid potential off by one mistakes, and move some
of the ugliest bits of the resolving code to a central place.

This is a transition patch, so there's no attempt at making the code
pretty. In particular we just provide a
setIntegerPosition()/integerPosition() pair, that are used throughout
the code to set/get the resolved positions. A follow-up patch will
move the rest of the resolver code in RenderGrid to the
GridResolvedPosition class, and we should be able to completely remove
those methods.

BUG=

[Xan, 2014-01-02 12:31:23]

In the end I decided to ignore all the extra operators to make the code "pretty"
and just make the smallest patch possible. In a follow-up and after moving all
the resolving code to the new class things should be a lot better.

[Manuel Rego, 2014-04-09 20:54:49]

I guess we can close this, as it has been already committed as part of
https://codereview.chromium.org/166623002/