Disable hermetic toolchain when building on macOS 10.14.

This is copied from https://chromium-review.googlesource.com/1333877
More info is available at crbug.com/904400.

Bug: webrtc:10093
Change-Id: Ia256b3515b354b501663f0536c2735542474d3c0
Reviewed-on: https://webrtc-review.googlesource.com/c/113422
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25932}
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index 669044d..582b3ef 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -41,8 +41,12 @@
   _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
                         [ target_os ],
                         "value")
-  assert(_result != 2,
-         "Do not allow building targets with the default " +
-             "hermetic toolchain if the minimum OS version is not met.")
-  use_system_xcode = _result == 0
+
+  # TODO(crbug.com/904400): Re-enable this once the hermetic toolchain runs on
+  # latest macOS release [Mojave].
+  # assert(_result != 2,
+  #        "Do not allow building targets with the default " +
+  #        "hermetic toolchain if the minimum OS version is not met.")
+  # use_system_xcode = _result == 0
+  use_system_xcode = _result != 1  # Remove when crbug.com/904400 is fixed.
 }