Made PositionInfo's capture explicit

Breaking out the Capture method and creating a separate nullary
constructor makes the magic more obvious and makes it easier to
create empty positions.

Change-Id: Idd174522f11620fbd11679c7cd8f770ada5bde7d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438820
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
6 files changed