mac: Let mac_sdk default to the oldest available SDK that's at least 10.6

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index c55f65e8d36a63962197b96e2789ccb17f57aaba..a88bd33c6066619cded224f40842226a1f50ac89 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -638,7 +638,7 @@
     # these defaults. If the SDK is installed someplace that Xcode doesn't
     # know about, set mac_sdk_path to the path to the SDK. If set to a
     # non-empty string, mac_sdk_path will be used in preference to mac_sdk.
-    'mac_sdk%': '10.6',
+    # mac_sdk gets its default value elsewhere in this file.
     'mac_sdk_path%': '',
     'mac_deployment_target%': '10.5',
 
@@ -1019,6 +1019,7 @@
           }],
 
           ['branding=="Chrome" and buildtype=="Official"', {
+            'mac_sdk%': '10.6',
             # Enable uploading crash dumps.
             'mac_breakpad_uploads%': 1,
             # Enable dumping symbols at build time for use by Mac Breakpad.
@@ -1026,6 +1027,7 @@
             # Enable Keystone auto-update support.
             'mac_keystone%': 1,
           }, { # else: branding!="Chrome" or buildtype!="Official"
+            'mac_sdk%': '<!(python mac/find_sdk.py 10.6)',
             'mac_breakpad_uploads%': 0,
             'mac_breakpad%': 0,
             'mac_keystone%': 0,