[LayoutNG] Add writing mode to NGDerivedConstraintSpace

Split NGDerivedConstraintSpace into a separate header and add direction
and writing mode fields. Also change it to use NGLogicalOffset and Size
instead of individual LayoutUnit fields for the offset and size values.

BUG=635619
R=cbiesinger@chromium.org

Review-Url: https://codereview.chromium.org/2236933003
Cr-Commit-Position: refs/heads/master@{#411389}
6 files changed