mac: Suppress partial availability warnings in theme helper.

mac: Suppress partial availability warnings in theme helper.

Moved the definition of a 10.7+ NSString constant into
mac/sdk_forward_declarations.h, and ensured that the NSString is explicitly
defined regardless of the OSX SDK version being compiled against.

BUG=471823
Committed: https://crrev.com/7e72bc9f6d57f34bc0a3697b3b2c261bf0fbea3d
Cr-Commit-Position: refs/heads/master@{#329459}

[erikchen, 2015-05-12 01:07:31]

erikchen@chromium.org changed reviewers:
+ avi@chromium.org, thakis@chromium.org

[erikchen, 2015-05-12 01:07:31]

thakis: Looking for an OWNER review of base/mac/sdk_forward_declarations.*
avi: Looking for an OWNER review of content/browser/theme_helper_mac.mm.

[Avi (use Gerrit), 2015-05-12 02:21:01]

lgtm

[Nico, 2015-05-12 03:14:08]

lgtm

[erikchen, 2015-05-12 18:42:41]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-05-12 18:43:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1133523005/1

[commit-bot: I haz the power, 2015-05-12 19:38:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-12 19:39:14]

Patchset 1 (id:??) landed as
https://crrev.com/7e72bc9f6d57f34bc0a3697b3b2c261bf0fbea3d
Cr-Commit-Position: refs/heads/master@{#329459}