Update developer documentation to recommend the “fetch” tool

doc/developing.ad

 // Copyright 2015 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 35 matching lines @@
    https://developer.apple.com/xcode/[Xcode], and for Windows, it’s
    https://www.visualstudio.com/[Visual Studio].
 
 == Getting the Source Code
 
 The main source code repository is a Git repository hosted at
 https://chromium.googlesource.com/crashpad/crashpad. Although it is possible to
 check out this repository directly with `git clone`, Crashpad’s dependencies are
 managed by
 https://dev.chromium.org/developers/how-tos/depottools#TOC-gclient[`gclient`]
-instead of Git submodules, so to work on Crashpad, it is best to use `gclient`
-to get the source code.
+instead of Git submodules, so to work on Crashpad, it is best to use `fetch` to
+get the source code.
 
-`gclient` is part of the
+`fetch` and `gclient` are part of the
 https://dev.chromium.org/developers/how-tos/depottools[depot_tools]. There’s no
-need to install it separately.
+need to install them separately.
 
 === Initial Checkout
 
 [subs="verbatim,quotes"]
 ----
-$ *mkdir \~/crashpad*
+$ *mkdir ~/crashpad*
 $ *cd ~/crashpad*
-$ *gclient config --unmanaged https://chromium.googlesource.com/crashpad/crashpad*
-$ *gclient sync*
+$ *fetch crashpad*
 ----
 
+`fetch crashpad` performs the initial `gclient sync`, establishing a
+fully-functional local checkout.
+
 === Subsequent Checkouts
 
 [subs="verbatim,quotes"]
 ----
 $ *cd ~/crashpad/crashpad*
 $ *git pull -r*
 $ *gclient sync*
 ----
 
 == Building
 
 Crashpad uses https://gyp.googlecode.com/[GYP] to generate
 https://martine.github.io/ninja/[Ninja] build files. The build is described by
 `.gyp` files throughout the Crashpad source code tree. The
 `build/gyp_crashpad.py` script runs GYP properly for Crashpad, and is also
-called when you run `gclient sync` or `gclient runhooks`.
+called when you run `fetch crashpad`, `gclient sync`, or `gclient runhooks`.
 
 The Ninja build files and build output are in the `out` directory. Both debug-
 and release-mode configurations are available. The examples below show the debug
 configuration. To build and test the release configuration, substitute `Release`
 for `Debug`.
 
 [subs="verbatim,quotes"]
 ----
 $ *cd ~/crashpad/crashpad*
 $ *ninja -C out/Debug*
@@ skipping 105 matching lines @@
 License Agreement] as appropriate before any submission can be accepted, and
 must be listed in the `AUTHORS` file. Contributors may be listed in the
 `CONTRIBUTORS` file.
 
 == Buildbot
 
 The https://build.chromium.org/p/client.crashpad/[Crashpad Buildbot] performs
 automated builds and tests of Crashpad. Before checking out or updating the
 Crashpad source code, and after checking in a new change, it is prudent to check
 the Buildbot to ensure that “the tree is green.”