mac: Update to Xcode 12.1

Still on the 10.15 SDK, so still no real change here.

I again locally removed libcodedirectory.dylib from the .yaml file
when running the script.

Tweak the instructions in the yaml file a bit. I used to copy over
the whole Xcode app, which used to take a few minutes. The new
process only takes seconds.

Also add a word of caution that following these instructions doesn't
update the locally used Xcode in itself.

Bug: 1108831
Change-Id: I3b3f3d698378c347161118ba59ce49a6e23ab891
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490920
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819638}

build/mac_toolchain.py

@@ -31,9 +31,9 @@ import sys
 # To build these packages, see comments in build/xcode_binaries.yaml
 MAC_BINARIES_LABEL = 'infra_internal/ios/xcode/xcode_binaries/mac-amd64'
 MAC_BINARIES_TAG = {
-    # This contains binaries from Xcode 12.0.1, along with the 10.15 SDK (aka
-    # 12A7300).
-    'default': '89j_tSMJj0_9hMbceEg-P-VmiY_4CeYwsPE1e_zzRPcC',
+    # This contains binaries from Xcode 12.1, along with the 10.15 SDK (aka
+    # 12A7403).
+    'default': '77fpfpUrA6kBF3yEaMBEvWHOomBWqoiRT3bEJ4bXxvUC',
     # This contains binaries from Xcode 12.2 beta 3, along with the
     # 11 SDK (aka 12B5035g).
     'xcode_12_beta': 'Hbv2yK-6odRW8p2nfq7EZyaOWRpcEiMn5mvZpG9exTwC',

build/xcode_binaries.yaml

 # This yaml file is used to package binaries from Xcode.app.
 # To use this:
-#   1) Move Xcode.app to the same directory as this file.
-#   2) Rename Xcode.app to xcode_binaries
-#   3) Call `cipd create --pkg-def build/xcode_binaries.yaml`
+#   1) Move Xcode.app to the same directory as this file,
+#      and rename Xcode.app to xcode_binaries. Or make a symlink:
+#      $ rm -rf build/xcode_binaries && \
+#        ln -s /Applications/Xcode.app build/xcode_binaries
+#   2) Call `cipd create --pkg-def build/xcode_binaries.yaml`
 # To deploy the newly created cipd package across the fleet, modify
 # mac_toolchain.py to point to the new cipd hash.
 #
+# Note that runhooks extracts the cipd file to build/mac/xcode_binaries -- your
+# build/xcode_binaries you're creating in step 1 above isn't used as part of
+# the Chromium build, build/mac_files/xcode_binaries is. So you need to
+# `runhooks` after updating the hash in mac_toolchain.py like everyone else to
+# get the new bits for your local build.
+#
 # The ACLs for this package are determined by the directory structure. The
 # nomenclature mirrors that of the hermetic toolchain to avoid ACL duplication.
 package: infra_internal/ios/xcode/xcode_binaries/mac-amd64