Use multidevice secondary text as description of button

Marks the "Set up" / "Verify" button as described-by the secondary text,
which contains additional context about the button

Bug: 1012565
Change-Id: I4c92615792e59177b33b3354557af14c5a3aa0f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917751
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715839}

chrome/browser/resources/settings/multidevice_page/multidevice_page.html

@@ -78,7 +78,8 @@
               if="[[shouldShowButton_(pageContentData)]]"
               restamp>
             <cr-button class="margin-matches-padding"
-                on-click="handleButtonClick_">
+                on-click="handleButtonClick_"
+                aria-describedby="multideviceSubLabel">
               [[getButtonText_(pageContentData)]]
             </cr-button>
           </template>