Fix xcode build.

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/300993002

build/standalone.gypi

diff --git a/build/standalone.gypi b/build/standalone.gypi
index 3b4f01b..bcc8cbc 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -246,4 +246,8 @@
       },  # target_defaults
     }],  # OS=="mac"
   ],
+  'xcode_settings': {
+    # See comment in Chromium's common.gypi for why this is needed.
+    'SYMROOT': '<(DEPTH)/xcodebuild',
+  }
 }
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