Convert GN visibility variables to lists.

Convert GN visibility variables to lists.

Currently this is either a list or a string. However, this is causing some problems because templates can't add to the invoker's visibility list without knowing if the original is a string or a list.

In an effort to make this consistent, I'm converting all visibiltiy to be lists, and will remove support for strings in a future build.

This exempts cld from header checking since it was confusing GN's header checker. It adds a ppapi header target as well that will be used by libyuv (that requires a roll).

TBR=scottmg

Committed: https://crrev.com/9af4262ad8020658ee04c07d13a2617af64147b1
Cr-Commit-Position: refs/heads/master@{#293638}

[brettw, 2014-09-06 00:17:46]

brettw@chromium.org changed reviewers:
+ scottmg@chromium.org

[scottmg, 2014-09-06 00:22:54]

lgtm

[brettw, 2014-09-06 16:10:56]

The CQ bit was checked by brettw@chromium.org

[commit-bot: I haz the power, 2014-09-06 16:12:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/brettw@chromium.org/544423002/20001

[commit-bot: I haz the power, 2014-09-06 16:18:44]

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

[commit-bot: I haz the power, 2014-09-06 16:18:47]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[brettw, 2014-09-06 21:15:18]

The CQ bit was checked by brettw@chromium.org

[commit-bot: I haz the power, 2014-09-06 21:17:16]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/brettw@chromium.org/544423002/20001

[commit-bot: I haz the power, 2014-09-06 21:22:58]

Committed patchset #2 (id:20001) as 1f64c97dfb962688257cd2129ce029b2ee9fe989

[commit-bot: I haz the power, 2014-09-10 03:43:28]

Patchset 2 (id:??) landed as
https://crrev.com/9af4262ad8020658ee04c07d13a2617af64147b1
Cr-Commit-Position: refs/heads/master@{#293638}