Copy FrameSelection.{cpp,h} to SelectionEditor.{cpp,h}

Copy FrameSelection.{cpp,h} to SelectionEditor.{cpp,h}

This patch copies "FrameSelection.{cpp,h}" to "SelectionEditor.{cpp,h}" as
a preparation of moving |FrameSelection::modify()| to |SelectionEditor| class
for inheriting change history.

This patch is a preparation of making selection to handle granularity for web
component, http://crrev.com/1277863002

BUG=513568
TEST=n/a; no behavior changes
NOTRY=true

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201858

[yosin_UTC9, 2015-09-07 05:28:26]

yosin@chromium.org changed reviewers:
+ hajimehoshi@chromium.org, yoichio@chromium.org

[yoichio, 2015-09-07 05:29:22]

lgtm

[hajimehoshi, 2015-09-07 05:31:55]

lgtm

[yosin_UTC9, 2015-09-07 05:38:13]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-07 05:38:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1331543002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1331543002/1

[commit-bot: I haz the power, 2015-09-07 05:42:41]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201858