Get rid of a redundant createLegacyEditingPosition() call in PositionAlgorithm<Strategy>::downstream

Get rid of a redundant createLegacyEditingPosition() call in PositionAlgorithm<Strategy>::downstream()

|createLegacyEditingPosition()|, which returns non-legacy editing position if
anchor node returns false for |editingIgnoresContent(node)|, call at L722 is
redundant because it takes a node object, which is |!editingIgnoresContent()|
by if-statement at L708.

This patch replaces a this redundant usage of |createLegacyEditingPosition()| to
|PositionAlgorithm<Strategy>| constructor for improving code health.

This patch is a similar to http://crrev.com/1262503010, which replaces
|createLegacyEditingPosition()| in |PositionAlgorithm<Strategy>::upstream()|.

This patch is a preparation of getting rid of legacy editing position.

BUG=513114
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=199948

[yosin_UTC9, 2015-08-04 05:36:54]

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

[yosin_UTC9, 2015-08-04 05:37:01]

PTAL

[tkent, 2015-08-04 06:47:59]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-08-04 06:47:59]

lgtm

[commit-bot: I haz the power, 2015-08-04 06:48:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1266413003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1266413003/1

[commit-bot: I haz the power, 2015-08-04 06:51:57]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=199948