feedback: Read "File will be sent to..." text once

Previously, https://crrev.com/c/1863363 changed the describedby tag so
that "File will be sent to google for debugging" is read along with the
upload selector. However, with that change, the text is also read when a
screen reader navigates to the next item. Mark the label as hidden, so
screen readers skip it when navigating. (But not when reading the
description of the "Choose File" button.)

TEST=Load onto an eve, navigate with screen reader.
BUG=chromium:1005528

Change-Id: If693cdc2f5cd434d5d4def7a1482399fd108d2f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869560
Commit-Queue: Miriam Zimmerman <mutexlox@chromium.org>
Reviewed-by: Ian Barkley-Yeung <iby@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: J Kardatzke <jkardatzke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707928}

chrome/browser/resources/feedback/html/default.html

@@ -56,7 +56,8 @@
       <div id="attach-error" class="attach-file-notification"
           role="alert" i18n-content="attach-file-to-big" hidden></div>
     </div>
-    <div id="attach-file-note" i18n-content="attach-file-note"></div>
+    <div id="attach-file-note" i18n-content="attach-file-note"
+      aria-hidden="true"></div>
     <!-- Screenshot -->
     <div id="screenshot-container" class="checkbox-field-container">
       <input id="screenshot-checkbox" type="checkbox"