Replace forward class declarations of VisiblePosition to include directives

Replace forward class declarations of VisiblePosition to include directive

This patch replaces forward class declarations |VisiblePosition| to include
directive of "VisiblePosition.h" as preparation of introducing
|VisiblePositionTemplate<Strategy>|, like |PositionAlgorithm<Strategy>|.

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=201056

[yosin_UTC9, 2015-08-24 04:57:30]

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

[yosin_UTC9, 2015-08-24 05:09:16]

PTAL

[tkent, 2015-08-24 05:45:02]

lgtm

[yosin_UTC9, 2015-08-24 05:48:45]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-24 05:48:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1306243006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1306243006/1

[commit-bot: I haz the power, 2015-08-24 07:14:35]

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

[commit-bot: I haz the power, 2015-08-24 07:14:36]

Try jobs failed on following builders:
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[yosin_UTC9, 2015-08-24 07:43:27]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-24 07:43:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1306243006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1306243006/1

[commit-bot: I haz the power, 2015-08-24 09:09:46]

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

[commit-bot: I haz the power, 2015-08-24 09:09:47]

Try jobs failed on following builders:
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[yosin_UTC9, 2015-08-24 09:13:56]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-24 09:14:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1306243006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1306243006/1

[commit-bot: I haz the power, 2015-08-24 10:03:13]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201056