Templatize extractSelectedText used by FrameSelection function

Templatize extractSelectedText used by FrameSelection function

This patch introduces a template function |extractSelectedTextAlgorithm<Strategy>|
to makes |extractSelectedText()| function used by |FrameSeleciton| to work
both on DOM tree and composed tree via |Strategy| template parameter.

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=197329

[yosin_UTC9, 2015-06-18 07:29:54]

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

[yosin_UTC9, 2015-06-18 07:29:55]

PTAL

[tkent, 2015-06-18 07:49:19]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-06-18 07:49:20]

lgtm

[commit-bot: I haz the power, 2015-06-18 07:50:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1184403004/1

[commit-bot: I haz the power, 2015-06-18 07:54:16]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197329