Use VisibilityState enum and move WebPageVisibilityState to public/platform.

Use VisibilityState enum and move WebPageVisibilityState to public/platform.

Actually, the enum will stay in public/web/ but a copy is in
public/platform until Chromium gets updated.

This is the first CL of a three-sided CL:
- <this>
- https://codereview.chromium.org/806703004
- https://codereview.chromium.org/831483003

Use VisibilityState enum in Document and SW's WindowClient.

BUG=447212
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=188151

[mlamouri (slow - plz ping), 2015-01-08 18:56:33]

mlamouri@chromium.org changed reviewers:
+ mkwst@chromium.org

[mlamouri (slow - plz ping), 2015-01-09 11:11:13]

Mike, I've updated the CL description to include the other parts. PTAL.

[Mike West, 2015-01-09 11:40:44]

LGTM.

[mlamouri (slow - plz ping), 2015-01-09 11:46:46]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-01-09 11:46:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/844733003/40001

[commit-bot: I haz the power, 2015-01-09 13:12:13]

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

[commit-bot: I haz the power, 2015-01-09 13:12:14]

Try jobs failed on following builders:
  linux_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/4...)

[mlamouri (slow - plz ping), 2015-01-09 13:38:16]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-01-09 13:39:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/844733003/40001

[commit-bot: I haz the power, 2015-01-09 14:23:35]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=188151