Move VisibleSelection::ChangeObserver out from VisibleSelection class

Move VisibleSelection::ChangeObserver out from VisibleSelection class

This patch moves |VisibleSelection::ChangeObserver| out from |VisibleSelection|
class as |VisibleSelectionChangeObserver| as preparation of introducing
composed tree version of |VisibleSelection| since it doesn't depend on
DOM tree traversal and composed tree traversal.

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=201785

[yosin_UTC9, 2015-09-04 07:10:33]

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

[yoichio, 2015-09-04 07:11:53]

lgtm

[yosin_UTC9, 2015-09-04 07:49:53]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-04 07:49:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1309413006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1309413006/1

[commit-bot: I haz the power, 2015-09-04 08:45:24]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201785