Add WebGPU gardener rotation to watchlist for expectations files

Tested using:

$ python3 third_party/depot_tools/watchlists.py webgpu-cts/expectations.txt
['webgpu-gardener@rotations.google.com']
$ python3 third_party/depot_tools/watchlists.py webgpu-cts/compat-expectations.txt
['webgpu-gardener@rotations.google.com']
$ python3 third_party/depot_tools/watchlists.py webgpu-cts/test_runner.js
[]

I don't know for sure if this will work, because the Gerrit UI doesn't
allow me to manually CC `webgpu-gardener@rotations.google.com` because
it's not a registered Gerrit account. However, I see some teams have
done similar things, so we can try it and see if it works.

Bug: none
Change-Id: I4f001b43f669bb7a35c026213137525d56a431bc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/205395
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>

WATCHLISTS

diff --git a/WATCHLISTS b/WATCHLISTS
new file mode 100644
index 0000000..cfb3d1b
--- /dev/null
+++ b/WATCHLISTS
@@ -0,0 +1,45 @@
+# Copyright 2024 The Dawn & Tint Authors
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+# 1. Redistributions of source code must retain the above copyright notice, this
+#    list of conditions and the following disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright notice,
+#    this list of conditions and the following disclaimer in the documentation
+#    and/or other materials provided with the distribution.
+#
+# 3. Neither the name of the copyright holder nor the names of its
+#    contributors may be used to endorse or promote products derived from
+#    this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+# Watchlist Rules
+# Refer: https://chromium.googlesource.com/chromium/src/+/main/docs/infra/watchlists.md
+
+# IMPORTANT: The regular expression filepath is tested against each path using
+# re.search, so it is not usually necessary to add .* at the end of the pattern.
+
+{
+    'WATCHLIST_DEFINITIONS': {
+        'expectations': {
+            'filepath': 'webgpu-cts/.*\.txt$',
+        }
+    },
+    'WATCHLISTS': {
+        'expectations': [
+            'webgpu-gardener@rotations.google.com',
+        ]
+    },
+}