Templatize canonicalPosition() in VisiblePosition

Templatize canonicalPosition() in VisiblePosition

This patch makes |canonicalPosition()| 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=197457

[yosin_UTC9, 2015-06-19 07:58:51]

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

[yosin_UTC9, 2015-06-19 08:13:20]

PTAL

[tkent, 2015-06-19 08:35:09]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-06-19 08:35:10]

lgtm

[commit-bot: I haz the power, 2015-06-19 08:35:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1194813002/1

[commit-bot: I haz the power, 2015-06-19 08:46:51]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197457