[ReaderMode] Fix theme selection announcements.

This change removes the label for each theme option, and instead uses
aria-label option element itself. As the label is not visually rendered
on the page, it is safe to be removed. This also fixes the issue of the
label being announced separately to the control itself when navigating
incrementally with VoiceOver.

Note: This change has no visual change, only affects screenreader
announcements.

AX-Relnotes:N/A

Fixed: 1087934
Change-Id: Ia7d063a03158ad8ca2f51a84b9535c427f7868cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2306138
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791153}

components/dom_distiller/core/html/dom_distiller_viewer.html

@@ -47,30 +47,18 @@ found in the LICENSE file.
                 </legend>
                 <ul>
                   <li class="theme-option">
-                    <label id="theme-option-light-label" class="screen-reader-only">
-                      light
-                    </label>
                     <input type="radio" name="theme" value="light"
-                       aria-labelledby="theme-option-light-label"
-                       id="theme-option-light" class="light" checked>
+                       aria-label="light" id="theme-option-light" class="light" checked>
                     <div class="checkmark"><i class="material-icons" aria-hidden="true">&#xE86c</i></div>
                   </li>
                   <li class="theme-option">
-                    <label id="theme-option-sepia-label" class="screen-reader-only">
-                      sepia
-                    </label>
                     <input type="radio" name="theme" value="sepia"
-                       aria-labelledby="theme-option-sepia-label"
-                       id="theme-option-sepia" class="sepia">
+                       aria-label="sepia" id="theme-option-sepia" class="sepia">
                     <div class="checkmark"><i class="material-icons" aria-hidden="true">&#xE86c</i></div>
                   </li>
                   <li class="theme-option">
-                    <label id="theme-option-dark-label" class="screen-reader-only">
-                      dark
-                    </label>
                     <input type="radio" name="theme" value="dark"
-                       aria-labelledby="theme-option-dark-label"
-                       id="theme-option-dark" class="dark">
+                       aria-label="dark" id="theme-option-dark" class="dark">
                     <div class="checkmark"><i class="material-icons" aria-hidden="true">&#xE86c</i></div>
                   </li>
                 </ul>