Add config to enable ARC on iOS/OS X to //build/config.

build/config/mac/BUILD.gn

Index: build/config/mac/BUILD.gn
diff --git a/build/config/mac/BUILD.gn b/build/config/mac/BUILD.gn
index af1fbbc092b050f00ac80637df0d2ecc688ebd28..78351b2dcc9cd9feabe1411dbe5b6b2f1a0aa82a 100644
--- a/build/config/mac/BUILD.gn
+++ b/build/config/mac/BUILD.gn
@@ -86,3 +86,13 @@ config("mac_executable_flags") {
   # Remove this when targeting >=10.7 since it is the default in that config.
   ldflags = [ "-Wl,-pie" ]  # Position independent.
 }
+
+# On Mac, some tools needs to be build with Mac OS X 10.9 or greater SDK.
+# Remove this and usage when the default SDK used on OS X is more recent
+# than that.

[erikchen, 2016/03/16 01:59:22]

Your comment doesn't match the behavior of your code.

"""
On Mac, some tools needs to be build with Mac OS X 10.9 or greater SDK.
"""

Currently, on Mac, everything should build against the 10.10 SDK.
-mmacosx-version-min is the deployment target (the lowest version of OSX that
the target will run on).

+config("macosx_version_min_10_9") {
+  common_flags = [ "-mmacosx-version-min=10.9" ]
+  ldflags = common_flags
+  cflags_objc = common_flags
+  cflags_objcc = common_flags
+}

[Dirk Pranke, 2016/03/16 01:34:30]

Is this actually used in this CL?

This config makes me a bit nervous, in that I don't much like the idea of
potentially doing a single toolchain build that produces targets that require
two different min versions of the OS. Also this flag is set by default on all
targets in the :runtime_library config, and I don't like having the flag set
twice on the command line if we can afford it.

Can we simply require macosx-version-min=10.9 if we're doing host toolchain
builds in an ios build?

Then we could change the default in //build_overrides/build.gni (which is pulled
in by mac_sdk.gni to create the $mac_deployment_target that is used).

[erikchen, 2016/03/16 01:59:22]

I assume this gets used in a private, downstream repository?

The flag could theoretically be set twice, with two different values, which
definitely seems wrong. Finding a way to update/override mac_deployment_target
seems like the right approach. 

[Dirk Pranke, 2016/03/16 02:25:38]

You're asking about the variable that is defined in //build_overrides/build.gni
? 

That's done so that we can set this in different upstream repos, actually, that
(re-)use //build (webrtc, in this case, which requires a newer version in their
standalone builds).

[sdefresne, 2016/03/16 08:40:43]

I don't like it either, but it is required to get iossim and class-dump to
compile. It is used in those two depends CLs, in those new files:
https://codereview.chromium.org/1806523002/diff/1/third_party/class-dump/BUIL...
https://codereview.chromium.org/1797253002/diff/1/testing/iossim/BUILD.gn

Neither tool depends on anything from Chromium, so I can instead override the
-mmac-osx-version-min in the tool config directly instead of putting it there in
the more common location. I did put it there in order to follow DRY, but if you
think it is more dangerous than beneficial, then I can remove it.