Add clarifying comment to device_policy_remover_generate sources

Clarify in a comment why the source is listed explicitly so future
readers are not tempted to remove it.

Bug: 933359
Test: none
Change-Id: I7f7b70bbe90853674e17c9a5b7ab5cc2016b2c96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1478877Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637877}

chrome/browser/chromeos/BUILD.gn

@@ -2695,6 +2695,10 @@ action("device_policy_remover_generate") {
       "$chrome_device_policy_pb2_dir/chrome_device_policy_pb2.py"
 
   sources = [
+    # It's important to explicitly list which files provided by this action's
+    # dependencies are actually used by this action, because the action is not
+    # automatically re-run on changes in one of the public_deps. See
+    # https://crbug.com/933359 for more information.
     chrome_device_policy_pb2_path,
   ]