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

This is a reland of fc86bd6c
We landed a workaround for the SDK bug, see 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=mark

Bug: 1098899
Change-Id: Idc42491929c5eb21a1f12c14ef3e9156fe6698e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315012Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791216}

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 (Universal) 12 beta, along with the
-    # 11 SDK (aka 12A8158a).
-    'xcode_12_beta': '_tvvMQXaruqACKkcaZmqHR_7S-S2pHrXgcjTWfbI1qoC',
+    # This contains binaries from Xcode 12 beta 3, along with the
+    # 11 SDK (aka 12A8169g).
+    'xcode_12_beta': 'hasRpexjGiBaGHjsib3eo73MQtTt3dRN1WdgyYk_m3MC',
 }
 
 # The toolchain will not be downloaded if the minimum OS version is not met.