Templatize PositionWithAffinity class

Templatize PositionWithAffinity class

Now |PositionWithAffinity| works only on DOM tree. This patch templatizes
|PositionWithAffinity| class and end enables it to allow other node-traversing
strategies, such as composed tree.

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=197247

[yosin_UTC9, 2015-06-17 08:02:37]

PTAL

[yosin_UTC9, 2015-06-17 08:02:50]

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

[tkent, 2015-06-17 08:15:49]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-06-17 08:15:49]

lgtm

[commit-bot: I haz the power, 2015-06-17 08:16:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187073014/20001

[commit-bot: I haz the power, 2015-06-17 09:39:53]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197247

[tasak, 2015-06-29 08:11:01]

tasak@google.com changed reviewers:
+ tasak@google.com

[tasak, 2015-06-29 08:11:02]

CORE_EXTERN_TEMPLATE in cpp will cause Android_compile_dbg compile failure when
using component build.

I will fix.

https://codereview.chromium.org/1187073014/diff/20001/Source/core/editing/Pos...
File Source/core/editing/PositionWithAffinity.cpp (right):

https://codereview.chromium.org/1187073014/diff/20001/Source/core/editing/Pos...
Source/core/editing/PositionWithAffinity.cpp:28: template class
CORE_EXTERN_TEMPLATE_EXPORT PositionWithAffinityTemplate<Position>;
Use CORE_TEMPLATE_EXPORT here.