Make StyleRareNonInheritedData::draggable_regions a bit field.

This patch changes the enum field draggable_regions in
StyleRareNonInheritedData a bit field so that it can be generated as
a keyword field.

The field is stored as an unsigned so that it can pack efficiently
on Windows.

BUG=628043

Review-Url: https://codereview.chromium.org/2912673002
Cr-Commit-Position: refs/heads/master@{#475448}
3 files changed