[NativeFS] Change settings exceptions site list to read only.

Actually being able to add/edit the list of blocked sites (other than removing
existing blocks) from chrome://settings/content/filesystem behaves weirdly
and doesn't actually match what other settings are doing. So instead use
a read-only list.

Bug: 999753, 999754
Change-Id: Ic06129dcaeda380cc734154dbf6926fb19618968
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779577Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692874}

chrome/browser/resources/settings/privacy_page/privacy_page.html

@@ -567,6 +567,7 @@
             </category-default-setting>
             <category-setting-exceptions
                 category="{{ContentSettingsTypes.NATIVE_FILE_SYSTEM_WRITE}}"
+                read-only-list
                 block-header="$i18n{siteSettingsBlock}"
                 search-filter="[[searchFilter_]]">
             </category-setting-exceptions>