mac: Update mac/arm docs a bit

- Mention that we now have a tester bot and link to it
- Put actual build instructions to top of the "Building" section
- Add some text about copying files to the DTK, and mention
  quarantine error messages, so that search engines can index them

Bug: none
Change-Id: I5b5f042280b6077ad89e217e4bf907f4b4c12fde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418984
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808608}

docs/mac_arm64.md

@@ -4,28 +4,18 @@ Chromium for arm Macs
 Apple is planning on selling Macs with arm chips by the end of 2020.
 This document describes the state of native binaries for these Macs.
 
-Building _for_ arm Macs
------------------------
-
-We're currently bringing up the build. At the moment, everything compiles
-in a vanilla release build (we haven't tried anything else) and Chromium
-starts up fine and seems to work. We haven't tried running tests yet.
-
-You can follow the [Mac-ARM64 label](https://crbug.com/?q=label%3Amac-arm64) to
-get updates on progress.
-
 There's a [bot](https://ci.chromium.org/p/chromium/builders/ci/mac-arm64-rel)
-that builds for arm that you can look at to get an idea of the current state. It
-cross-builds on an Intel machine, and we don't have enough hardware to
-continuously run tests.
+that builds for arm. It cross-builds on an Intel machine.
 
-As an alternative to building locally, changes can be submitted to the opt-in
-[mac-arm64-rel
-trybot](https://ci.chromium.org/p/chromium/builders/try/mac-arm64-rel). A small
-number of [swarming bots](https://goto.corp.google.com/run-on-dtk) are also
-available for Googlers to run tests on.
+There's also a [tester
+bot](https://ci.chromium.org/p/chromium/builders/ci/mac-arm64-rel-tests)
+that continuously runs tests. Most tests pass.
 
-To build for arm64, you have to do 2 things:
+Building _for_ arm Macs
+-----------------------
+
+You can build Chrome for arm macs on an Intel Mac. To build for arm64, you have
+to do 2 things:
 
 1. use the `MacOSX11.0.sdk` that comes with
    Xcode 12 beta. If you're on Google's corporate network, edit your `.gclient`
@@ -33,8 +23,8 @@ To build for arm64, you have to do 2 things:
 
        "custom_vars": { "mac_xcode_version": "xcode_12_beta" },
 
-   Then just `gclient sync` and you'll automatically get that SDK and will build
-   with it.
+   Then just run `gclient sync` and you'll automatically get that SDK and will
+   build with it.
 
    Otherwise, manually download and install the current Xcode 12 beta and make
    it the active Xcode with `xcode-select`.
@@ -43,6 +33,32 @@ To build for arm64, you have to do 2 things:
 
 Then build normally.
 
+To run a built Chromium, you need to copy it to your arm mac. If you don't
+do a component build (e.g. a regular `is_debug=false` build), you can just
+copy over Chromium.app from your build directory. If you copy it using
+macOS's "Shared Folder" feature and Finder, Chromium.app should be directly
+runnable. If you zip, upload Chromium.app to some web service and download
+it on the DTK, browsers will set the `com.apple.quarantine` bit, which will
+cause Finder to say `"Chromium" is damanged and can't be opened. You should
+move it to the Trash."`. In Console.app, the kernel will log
+`kernel: Security policy would not allow process: 2204,
+/Users/you/Downloads/Chromium.app/Contents/MacOS/Chromium` and amfid will log
+`amfid: /Users/you/Downloads/Chromium.app/Contents/MacOS/Chromium signature not
+valid: -67050`. To fix this, open a terminal and run
+
+    % cd ~/Downloads && xattr -rc Chromium.app
+
+After that, it should start fine.
+
+As an alternative to building locally, changes can be submitted to the opt-in
+[mac-arm64-rel
+trybot](https://ci.chromium.org/p/chromium/builders/try/mac-arm64-rel). A small
+number of [swarming bots](https://goto.corp.google.com/run-on-dtk) are also
+available for Googlers to run tests on.
+
+You can follow the [Mac-ARM64 label](https://crbug.com/?q=label%3Amac-arm64) to
+get updates on progress.
+
 Building _on_ arm Macs
 -----------------------