[css-grid] Rename GridCoordinate to GridArea

As the comment in GridCoordinate says,
it actually represents a grid area as it stores
the initial and final positions in both axis (columns and rows).

Someone can think about a grid coordinate just like a single cell.
However this class was representing an area of several cells.

On top of that the "grid area" concept is defined in the spec:

No new tests, no change of behavior.


Review URL: https://codereview.chromium.org/1648333002

Cr-Commit-Position: refs/heads/master@{#372638}
9 files changed