Templatize functions in htmlediting.cpp

Templatize functions in htmlediting.cpp

This patch templatizes functions in "htmlediting.cpp", which are called during
constructing |VisblePositon|, for a preparation of making |VisiblePosition|
class to work with position in composed tree:
 - lastEditablePositionBeforePositionInRoot()
 - nextCandidate()
 - previousCandidate()
 - previousVisuallyDistinctCandidate()

This patch is result of collaboration work with hajimehoshi@chromium.org
for selection of web components. After this patch, we are going to
implement another strategies for the composed tree. We've already
prepared that at http://crrev.com/1106433002.

BUG=275851
TEST=n/a; No behavior changes.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197330

[yosin_UTC9, 2015-06-18 04:28:01]

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

[yosin_UTC9, 2015-06-18 05:26:40]

PTAL

mac_blink_rel bot failure doesn't relate this patch.

[yosin_UTC9, 2015-06-18 07:29:14]

yosin@chromium.org changed reviewers:
+ tkent@chromium.org
- yoichio@chromium.org

[yosin_UTC9, 2015-06-18 07:29:15]

PTAL

[yosin_UTC9, 2015-06-18 07:30:36]

mac_blink_rel bot failure doesn't relate to this patch.

[tkent, 2015-06-18 07:45:52]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-06-18 07:45:52]

lgtm

[commit-bot: I haz the power, 2015-06-18 07:46:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174183004/20001

[commit-bot: I haz the power, 2015-06-18 08:28:34]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197330