Move serializer related files in core/editing/ related files into core/editing/serializers/

Move serializer related files in core/editing/ related files into core/editing/serializers/ 

This patch moves spell checker related files into "core/editing/serializers/"
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:
 - HTMLInterchange.{cpp,h}
 - MarkupAccumulator.{cpp,h}
 - MarkupFormatter.{cpp,h}
 - Serialization.{cpp,h}
 - StyledMarkupAccumulator.{cpp,h}
 - StyledMarkupSerializer.{cpp,h}
 - StyledMarkupSerializerTest.cpp
 - TextOffset.{cpp,h}

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200633

[yosin_UTC9, 2015-08-17 04:47:36]

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

[yosin_UTC9, 2015-08-17 04:48:16]

PTAL

[tkent, 2015-08-17 05:32:30]

lgtm

[yosin_UTC9, 2015-08-17 05:35:31]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-17 05:35:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1295073002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1295073002/20001

[commit-bot: I haz the power, 2015-08-17 05:44:10]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-17 05:44:11]

Try jobs failed on following builders:
  android_chromium_gn_compile_dbg on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_chromiu...)

[yosin_UTC9, 2015-08-17 06:30:07]

The CQ bit was checked by yosin@chromium.org

[yosin_UTC9, 2015-08-17 06:30:08]

The patchset sent to the CQ was uploaded after l-g-t-m from tkent@chromium.org
Link to the patchset: https://codereview.chromium.org/1295073002/#ps40001
(title: "2015-08-17T15:24:31 Rebase for Source/web/WebPageSerializerImpl.cpp")

[commit-bot: I haz the power, 2015-08-17 06:30:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1295073002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1295073002/40001

[commit-bot: I haz the power, 2015-08-17 07:57:44]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200633