Templatize canonicalizeCandidate() in VisiblePosition

Templatize canonicalizeCandidate() in VisiblePosition

This patch makes |canonicalizeCandidate()| in |VisiblePosition| to a template
function with template parameter |PositionType|, which takes either |Position|
for DOM tree or |PositionInComposedTree|.

This patch is a preparation to make |VisiblePositon| to work with a position
in composed tree for selection for web component.

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=197447

[yosin_UTC9, 2015-06-19 02:11:26]

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

[yosin_UTC9, 2015-06-19 04:00:53]

PTAL

[tkent, 2015-06-19 04:05:08]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-06-19 04:05:08]

lgtm

[commit-bot: I haz the power, 2015-06-19 04:05:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1193783002/20001

[commit-bot: I haz the power, 2015-06-19 06:44:47]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197447