Fix up DIR_METADATA for supervised_user directories

* Remove spaces from component name
* Do not inherit from multiple COMMON_METADATA files when fields will
  clobber
* Add missing files (e.g.  //components/supervised_user/DIR_METADATA)

Bug: None
Change-Id: Ic19a2513e15b7ee30d92edc264569fa2e0e3f6f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4262089
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1108157}

components/safe_search_api/DIR_METADATA

@@ -1,7 +1,5 @@
-# Enterprise
+# This first mixin gets clobbered by the second one, but exists
+# for you, the reader, to know that this directory is co-owned by
+# the "Enterprise" component.
 mixins: "//components/policy/COMMON_METADATA"
-# Chrome Browser
-mixins: "//chrome/browser/supervised_user/android/COMMON_METADATA"
-# ChromeOS
-mixins: "//chrome/browser/supervised_user/chromeos/COMMON_METADATA"
-
+mixins: "//components/supervised_user/COMMON_METADATA.chromeos"