[CBCM] Add comment in PolicyFetchRequest proto

Adds a comment to the PolicyFetchRequest.policy_type proto field to
document the correct value to use for iOS CBCM policy fetch requests.

Change-Id: I32e720e5cd028d93dc0f0b39904e500500f2a251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618198Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#841523}

components/policy/proto/device_management_backend.proto

@@ -363,6 +363,7 @@ message PolicyFetchRequest {
   //   google/chromeos/user => ChromeSettingsProto
   //   google/chromeos/publicaccount => ChromeSettingsProto
   //   google/chrome/machine-level-user => ChromeSettingsProto
+  //   google/chrome/machine-level-user-ios => ChromeSettingsProto
   //   google/chrome/extension => ExternalPolicyData
   //   google/chrome/machine-level-extension => ExternalPolicyData
   //   google/chromeos/signinextension => ExternalPolicyData