Move global function lowestEditableAncestor() to local function in VisibleSelection.cpp

Move global function lowestEditableAncestor() to local function in VisibleSelection.cpp

This patch moves global function |lowestEditableAncestor()| in
"EditingUtilities.{cpp,h}" to local function in "VisibleSelection.cpp" to
narrow visibility for improving code health and ease of implement composed tree
version.

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201302

[yosin_UTC9, 2015-08-27 08:28:30]

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

[yosin_UTC9, 2015-08-27 08:29:48]

PTAL

[hajimehoshi, 2015-08-27 08:49:35]

lgtm

[yosin_UTC9, 2015-08-27 08:50:04]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-27 08:50:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1318943002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1318943002/1

[commit-bot: I haz the power, 2015-08-27 09:31:31]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201302