Use PositionWithAffinity as internal representation of VisiblePostion

Use PositionWithAffinity as internal representation of VisiblePostion

This patch replaces member variables |Position| and |TextAffinity| in
|VisiblePostion| class to |PositionWithAffinity| to simplify construction
of an object for preparation of templatizing |VisiblePostion| to introduce
composed tree version.

This patch also moves default constructor into .cpp file from header file to
reduce size of header file.

This patch is a preparation of making selection to handle granularity for web
component, http://crrev.com/1277863002

BUG=513568
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201619

[yosin_UTC9, 2015-09-02 05:44:16]

yosin@chromium.org changed reviewers:
+ hajimehoshi@chromium.org, yoichio@chromium.org

[yosin_UTC9, 2015-09-02 06:00:18]

PTAL

[yoichio, 2015-09-02 06:38:25]

lgtm

[hajimehoshi, 2015-09-02 06:39:13]

Impressive. lgtm

[yosin_UTC9, 2015-09-02 06:39:38]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-02 06:39:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311913006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311913006/1

[commit-bot: I haz the power, 2015-09-02 06:45:49]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201619