Make exceptions in Flash settings page read only.

Flash content settings are planned to become ephemeral and this feature
is behind 'EnableEphemeralFlashPermission' switch.
To make the ephemeral case more apparent to the user, the 'Add' buttons
for 'Allow' and 'Block' exceptions in Flash content settings are
removed and this is done by making the list read only.

Bug: 850062
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I0c216cc0f319a579bcab1639f0b7345070b474c8
Reviewed-on: https://chromium-review.googlesource.com/1124360Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572470}

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

@@ -392,7 +392,8 @@
 </if>
           <category-setting-exceptions
               category="{{ContentSettingsTypes.PLUGINS}}"
-              block-header="$i18n{siteSettingsBlock}">
+              block-header="$i18n{siteSettingsBlock}"
+              read-only-list="[[enableEphemeralFlashPermission_]]">
           </category-setting-exceptions>
         </settings-subpage>
       </template>