Fix NeedsFullRepaintForPositionedMovementLayout

Fix NeedsFullRepaintForPositionedMovementLayout

The logic was just plain wrong: during a position movement layout,
we have to invalidate the old and new position as we are shifted.
The current code would generate incremental invalidation as it
fell down the wrong path.

Refactored the code to do what it's supposed to do and added
an invalidation test to cover a case where incremental
invalidation failed.

BUG=352050
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169505

[Julien - ping for review, 2014-03-17 21:20:38]

First bug where removing invalidation exposes an underlying issue in the code.
Simple review!

[leviw_travelin_and_unemployed, 2014-03-17 21:58:36]

Makes sense to me. LGTM.

[Julien - ping for review, 2014-03-17 22:31:02]

The CQ bit was checked by jchaffraix@chromium.org

[commit-bot: I haz the power, 2014-03-17 22:31:13]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jchaffraix@chromium.org/202433003/20001

[commit-bot: I haz the power, 2014-03-18 03:23:22]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-18 03:23:23]

Commit queue rejected this change because the description was changed
between the time the change entered the commit queue and the time it
was ready to commit. You can safely check the commit box again.

[Julien - ping for review, 2014-03-19 00:17:47]

The CQ bit was checked by jchaffraix@chromium.org

[commit-bot: I haz the power, 2014-03-19 00:17:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jchaffraix@chromium.org/202433003/20001

[commit-bot: I haz the power, 2014-03-19 02:44:09]

Change committed as 169505