Forward-declare enums in a way that is compatible with NS_ENUM

Forward-declare enums in a way that is compatible with NS_ENUM

Starting with the 10.8 SDK and the iOS 6.0 SDK, enums are declared
strongly typed in C++11 mode, which the old forward-declares weren't
compatible with. This creates a new macro for forward-declaring these
enums in a way that will match the SDK and compilation mode.

BUG=226494
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=192543

[stuartmorgan, 2013-04-04 17:05:01]

I simplified slightly from the SDK's ifdef; it also checks toolchain version and
has handling of non-C++ compilation, but I assume we don't care about either.

[Nico, 2013-04-04 17:07:58]

lgtm

(Do you want to have some tracking bug for turning on c++11 for chrome/ios?)

[stuartmorgan, 2013-04-04 17:18:49]

On 2013/04/04 17:07:58, Nico wrote:
> (Do you want to have some tracking bug for turning on c++11 for chrome/ios?)

Done.

PTAL, because I'm an idiot and didn't test in a non-c++11 iOS compile before
uploading. We don't want to check __has_feature(objc_fixed_enum) because that's
always true regardless of our compilation mode.

[Nico, 2013-04-04 17:20:53]

still lgtm

[commit-bot: I haz the power, 2013-04-05 07:01:21]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/stuartmorgan@chromium.org/13455004/3001

[commit-bot: I haz the power, 2013-04-05 10:45:09]

Change committed as 192543