mac: Make 'clang=1' the default for official builds.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 8fb4dfa267e4f99bb1155973a3cce9a86a28be0b..cce352b1e6119d68125d8b6222e900cd6dc4f252 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -557,6 +557,14 @@
           ['branding=="Chrome" and buildtype=="Official"', {
             'mac_breakpad%': 1,
             'mac_keystone%': 1,
+
+            # Official builds use clang, but only on m15+. Since there's just
+            # one buildbot config for the builder for m13, m14, and m15, this
+            # can't be defined in the buildbot config but is instead defined
+            # here (it was added after the m14 branch was cut). This is in the
+            # buildtype=="Official" section so that developers don't see it
+            # for their local builds.
+            'clang%': 1,
           }, { # else: branding!="Chrome" or buildtype!="Official"
             'mac_breakpad%': 0,
             'mac_keystone%': 0,