Temporarily allow toolchain/OS mismatch to fallback on local toolchain.

Historically, we've disallowed toolchain/OS version mismatches to fallback on
local toolchain. This prevents bots from silently succeeding [but building with
the wrong toolchain].

Since the hermetic toolchain currently does not run on the
latest OS version, this check is also affecting all developers, which we'd like
to avoid. This CL temporarily disables the check.

Bug: 904400
Change-Id: I4a3edc1218b50017a1dba96b54088986f48a6862
Reviewed-on: https://chromium-review.googlesource.com/c/1333877Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607695}

build_overrides/build.gni

@@ -50,8 +50,11 @@ if (host_os == "mac" && use_system_xcode == "") {
   _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.")
+
+  # TODO(https://crbug.com/904400): Re-enable this check once 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
 }