Make startOfParagraph() not to use Position::deprecatedEditingOffset()

Make startOfParagraph() not to use Position::deprecatedEditingOffset()

This patch replaces |Position::deprecatedEditingOffset()| call in
|startOfParagraph()| by |Position::computeEditingOffset()|, since it is used
for computing offset of |VisiblePosition| which can not be middle of editing
ignore content, for improving code health.

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=199887

[yosin_UTC9, 2015-08-03 06:35:55]

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

[yosin_UTC9, 2015-08-03 06:47:16]

PTAL

[tkent, 2015-08-03 07:08:55]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-08-03 07:08:56]

lgtm

[commit-bot: I haz the power, 2015-08-03 07:09:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1258313006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1258313006/1

[commit-bot: I haz the power, 2015-08-03 08:38:18]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=199887