Support Reclient developer builds

Step 0) Run gclient runhooks

- Add `download_remoteexec_cfg: True` in `custom_vars` of your .gclient.
- `fetch_reclient_cfgs` hook needs to run.

Step 1) Generate build dir

❯ gn gen out/rbe --args="use_remoteexec=true"

Step 2)

❯ autoninja -C out/rbe all
Proxy started successfully.
ninja: Entering directory `out/rbe'
[0/1] Regenerating ninja files
[8776/8776] STAMP obj/default.stamp
Shutting down reproxy...
RBE Stats: ↓ 693.96 MB, ↑ 25.51 MB, 6474 remote executions

Bug: b/243595573
Change-Id: I32c3e0706effc45ac8ca8b882fbcdc71171b53d2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/311620
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Auto-Submit: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#40444}

DEPS

@@ -35,6 +35,10 @@ vars = {
   'checkout_fuchsia_boot_images': "terminal.qemu-x64",
   'checkout_fuchsia_product_bundles': '"{checkout_fuchsia_boot_images}" != ""',
 
+  # Fetch configuration files required for the 'use_remoteexec' gn arg
+  'download_remoteexec_cfg': False,
+  # RBE instance to use for running remote builds
+  'rbe_instance': 'projects/rbe-webrtc-developer/instances/default_instance',
   # reclient CIPD package version
   'reclient_version': 're_client_version:0.109.0.927890d-gomaip',
 
@@ -2693,6 +2697,21 @@ hooks = [
                 '-vpython-tool', 'install',
     ],
   },
+  # Download remote exec cfg files
+  {
+    'name': 'fetch_reclient_cfgs',
+    'pattern': '.',
+    'condition': 'download_remoteexec_cfg',
+    'action': ['python3',
+               'src/buildtools/reclient_cfgs/fetch_reclient_cfgs.py',
+               '--rbe_instance',
+               Var('rbe_instance'),
+               '--reproxy_cfg_template',
+               'reproxy.cfg.template',
+               '--quiet',
+               '--hook',
+               ],
+  },
 ]
 
 recursedeps = []