[Password Manager] Export progress dialog dismissable only via cancel button

As per mocks
https://docs.google.com/presentation/d/1nIm5OmaOnb85ZAwMZPSVqHT0vkFbsRYZhOzbmznWU_c/edit#slide=id.g284db53a0f_0_226

Bug: 789561
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I504755e6f362e130a59c806ed700866df5ef06f4
Reviewed-on: https://chromium-review.googlesource.com/924065
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537631}

chrome/browser/resources/settings/passwords_and_forms_page/passwords_export_dialog.html

@@ -35,7 +35,7 @@
       </div>
     </dialog>
 
-    <dialog is="cr-dialog" id="dialog_progress" close-text="$i18n{close}">
+    <dialog is="cr-dialog" id="dialog_progress" no-cancel="true">
       <div slot="title">$i18n{exportingPasswordsTitle}</div>
       <div slot="body">
         <paper-progress indeterminate class="blue"></paper-progress>