Refer to build/mac/find_sdk.py in a more robust way.

This broke the WebKit Mac bot, as the directory it works in is different there
compared to the Chromium configuration. Determine the source's root through the
DEPTH define, and then walk up in the build/mac/ directory.

BUG=
TEST=WebKit Mac builder doesn't break after a Chromium DEPS roll.


Review URL: https://chromiumcodereview.appspot.com/10824105

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149181 0039d316-1c4b-4281-b951-d872f2087c98

build/common.gypi

@@ -1028,7 +1028,7 @@
             # Enable Keystone auto-update support.
             'mac_keystone%': 1,
           }, { # else: branding!="Chrome" or buildtype!="Official"
-            'mac_sdk%': '<!(python mac/find_sdk.py 10.6)',
+            'mac_sdk%': '<!(python <(DEPTH)/build/mac/find_sdk.py 10.6)',
             'mac_breakpad_uploads%': 0,
             'mac_breakpad%': 0,
             'mac_keystone%': 0,