Templatize version updateSelectionForMouseDrag in SelectionController

Templatize version updateSelectionForMouseDrag in SelectionController

This patch introduces a template function |updateSelectionForMouseDragAlgorithm<Strategy>|
for a preparation of making |updateSelectionForMouseDrag()| to work with both
DOM tree and composed tree.

Following patch will make |expandSelectionToRespectUserSelectAll()| to check
runtime flag |SelectionForWebComponents| and work for either positions in DOM
tree or positions in 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=197454

[yosin_UTC9, 2015-06-19 06:56:26]

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

[yosin_UTC9, 2015-06-19 07:42:03]

PTAL

[tkent, 2015-06-19 07:56:23]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-06-19 07:56:23]

lgtm

[commit-bot: I haz the power, 2015-06-19 07:56:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1193813003/20001

[commit-bot: I haz the power, 2015-06-19 08:17:32]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197454