Revert "mac/arm: Update to Xcode 12 beta 3."

This reverts commit fc86bd6c.

Reason for revert: https://crbug.com/1108561

Original change's description:
> mac/arm: Update to Xcode 12 beta 3.
> 
> Bug: 1098899
> Change-Id: I9423e63db7f55fe4c18271a26ff60cee6b802bc2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314676
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Commit-Queue: Mark Mentovai <mark@chromium.org>
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#791026}

TBR=thakis@chromium.org,mark@chromium.org

Change-Id: Ie6cdf343a6ed9530f3318530d50831f6a3197cce
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1098899
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315480Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791090}

build/mac_toolchain.py

@@ -34,9 +34,9 @@ MAC_BINARIES_TAG = {
     # This contains binaries from Xcode 11.2.1, along with the 10.15 SDKs (aka
     # 11B53).
     'default': 'X5ZbqG_UKa-N64_XSBkAwShWPtzskeXhQRfpzc_1KUYC',
-    # This contains binaries from Xcode 12 beta 3, along with the
-    # 11 SDK (aka 12A8169g).
-    'xcode_12_beta': 'hasRpexjGiBaGHjsib3eo73MQtTt3dRN1WdgyYk_m3MC',
+    # This contains binaries from Xcode (Universal) 12 beta, along with the
+    # 11 SDK (aka 12A8158a).
+    'xcode_12_beta': '_tvvMQXaruqACKkcaZmqHR_7S-S2pHrXgcjTWfbI1qoC',
 }
 
 # The toolchain will not be downloaded if the minimum OS version is not met.