webui: make cr-fingerprint-progress-arc images aria-hidden

Screenreaders currently announce the chrome://... resource reference
when focusing these icons. The dialog embedding this component already
has labels that provide the semantic context.

Bug: 974046
Change-Id: If0850187ab2bce95b8bc8756f3ab7693df7a27d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1938256
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Auto-Submit: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#722595}

ui/webui/resources/cr_elements/cr_fingerprint/cr_fingerprint_progress_arc.html

@@ -61,12 +61,16 @@
       <canvas id="canvas" height="240" width="460"></canvas>
     </div>
     <div id="imageDiv">
-      <img id="scanningAnimation" src="chrome://theme/IDR_FINGERPRINT_ICON_ANIMATION">
+      <img id="scanningAnimation"
+          src="chrome://theme/IDR_FINGERPRINT_ICON_ANIMATION"
+          aria-hidden="true">
       <iron-icon id="enrollmentDone" icon="cr-fingerprint-icon:enrollment-done" hidden>
       </iron-icon>
     </div>
     <div id="checkmarkDiv" hidden>
-      <img id="checkmarkAnimation" src="chrome://theme/IDR_FINGERPRINT_COMPLETE_TICK">
+      <img id="checkmarkAnimation"
+          src="chrome://theme/IDR_FINGERPRINT_COMPLETE_TICK"
+          aria-hidden="true">
     </div>
   </template>
   <script src="cr_fingerprint_progress_arc.js"></script>