Reland "mac: roll to 10.15 SDK"

This is a reland of fd84ab8a

Original change's description:
> mac: roll to 10.15 SDK
>
> This change rolls the hermetic Xcode to 11.2.1 aka 11B53, with the
> 10.15 SDK.
>
> Bug: 1023913
> Change-Id: I072a57b5f9c93bc08984b05d0de3ea62886a59dd
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1964531
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
> Cr-Original-Commit-Position: refs/heads/master@{#725141}
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993723
> Cr-Commit-Position: refs/heads/master@{#730238}

Bug: 1023913
Change-Id: Icb5ca715edd7972c8b3824aa58a8e2b8df25d15b
Tbr: thakis@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2022257Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735493}

build/config/mac/mac_sdk.gni

@@ -35,7 +35,7 @@ declare_args() {
   # The SDK version used when making official builds. This is a single exact
   # version, not a minimum. If this version isn't available official builds
   # will fail.
-  mac_sdk_official_version = "10.14"
+  mac_sdk_official_version = "10.15"
 }
 
 # Check that the version of macOS SDK used is the one requested when building

build/mac_toolchain.py

@@ -28,10 +28,10 @@ import subprocess
 import sys
 
 
-# This contains binaries from Xcode 10.12.1, along with the 10.14 SDKs. To build
-# this package, see comments in build/xcode_binaries.yaml
+# This contains binaries from Xcode 11.2.1, along with the 10.15 SDKs (aka
+# 11B53). To build this package, see comments in build/xcode_binaries.yaml
 MAC_BINARIES_LABEL = 'infra_internal/ios/xcode/xcode_binaries/mac-amd64'
-MAC_BINARIES_TAG = 'yjQtk3auAegQO4t18uBtBlKbj76xBjVtLE-3UM2faRUC'
+MAC_BINARIES_TAG = 'HHSqYjco5P_Q5g9gOWV6djGpZ9aqlV12N0hxKJv8U3gC'
 
 # The toolchain will not be downloaded if the minimum OS version is not met.
 # 17 is the major version number for macOS 10.13.