Move spell checker in core/editing/ related files into core/editing/spellcheck/

Move spell checker in core/editing/ related files into core/editing/spellcheck/ 

This patch moves spell checker related files into "core/editing/spellcheck/"
directory to reduce number of files immediately under "core/editing/" for ease
of working these files as same as issue 448772, which moves iterator related
files into "core/editing/iterators/".

Moved files:
 - SpellCheckRequest.{cpp,h}
 - SpellChecker.{cpp,h}
 - TextCheckingHelper.{cpp,h}

BUG=509527
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200625

[yosin_UTC9, 2015-08-17 02:00:46]

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

[yosin_UTC9, 2015-08-17 02:01:01]

PTAL

[tkent, 2015-08-17 02:15:50]

lgtm

[yosin_UTC9, 2015-08-17 02:18:40]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-17 02:18:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287263004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287263004/1

[commit-bot: I haz the power, 2015-08-17 03:22:37]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200625