Move GridCoordinate and GridSpan to a separate file

Move GridCoordinate and GridSpan to a separate file

In order to implement 'grid-template', we need a way to describe a
grid area's coordinates. That's what GridCoordinate is about and
instead of duplicating code, splitting the classes will enable code
sharing.

While moving the code, added some comments about the 2 classes.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=153427

[Julien - ping for review, 2013-07-02 00:46:02]

Very simple refactoring for the taker!

[eseidel, 2013-07-02 03:35:49]

lgtm

[commit-bot: I haz the power, 2013-07-02 03:36:04]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jchaffraix@chromium.org/18345011/3001

[commit-bot: I haz the power, 2013-07-02 04:31:26]

Retried try job too often on linux_layout_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_layo...

[commit-bot: I haz the power, 2013-07-02 14:40:15]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jchaffraix@chromium.org/18345011/3001

[commit-bot: I haz the power, 2013-07-02 15:16:05]

Change committed as 153427