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

This reverts commit b203a35a.

Reason for revert: suspected to break some browser_tests: see
- https://findit-for-me.appspot.com/waterfall/failure?url=https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29/builds/82842
- https://findit-for-me.appspot.com/waterfall/failure?url=https://build.chromium.org/p/chromium.memory/builders/Linux%20CFI/builds/14865

Original change's description:
> [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/+/1779577
> Reviewed-by: Dan Beam <dbeam@chromium.org>
> Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#692874}

TBR=dbeam@chromium.org,mek@chromium.org

Change-Id: I0394389f87d9640b7e76b9f9460046c45aea88aa
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 999753, 999754
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782375Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693013}

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

@@ -567,7 +567,6 @@
             </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>