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

This reverts commit 0f39ef65.

Reason for revert: unrelated to the failures, test is still failing after the revert

Original change's description:
> 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/+/1782375
> Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
> Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#693013}

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

Change-Id: Ib6e119881662967f84f4e4211c7bbd64911c8dd3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 999753, 999754
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783715Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693020}

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>