Add the partial screen magnifier to Chrome OS.

chrome/browser/resources/options/browser_options.html

 <div id="settings" class="page" hidden>
   <header>
     <h1 i18n-content="settingsTitle"></h1>
   </header>
 <if expr="not pp_ifdef('chromeos')">
   <include src="sync_section.html">
 </if>
 <if expr="pp_ifdef('chromeos')">
   <section>
     <h3 i18n-content="sectionTitleInternet"></h3>
@@ skipping 499 matching lines @@
         <div class="checkbox">
           <label>
             <input id="accessibility-high-contrast-check" type="checkbox">
             <span i18n-content="accessibilityHighContrast"></span>
           </label>
         </div>
       </div>
       <div class="option-name">
         <div class="checkbox">
           <label>
-            <input id="accessibility-screen-magnifier-check" type="checkbox">
             <span i18n-content="accessibilityScreenMagnifier"></span>
+            <select id="accessibility-screen-magnifier-type">
+              <option value="off"
+               i18n-content="accessibilityScreenMagnifierOff"></option>
+              <option value="full"
+               i18n-content="accessibilityScreenMagnifierFull">
+              </option>
+              <option value="partial"
+               i18n-content="accessibilityScreenMagnifierPartial">
+              </option>
+            </select>
           </label>
         </div>
       </div>
     </div>
   </section>
 <if expr="pp_ifdef('chromeos')">
   <section id="factory-reset-section" hidden>
     <h3 i18n-content="factoryResetTitle"></h3>
     <div>
       <span class="settings-row" i18n-content="factoryResetDescription">
@@ skipping 18 matching lines @@
   </section>
 </if>
 </div>  <!-- advanced-settings-container -->
 </div>  <!-- advanced-settings -->
 <footer>
   <button id="advanced-settings-expander" class="link-button"
       i18n-content="showAdvancedSettings">
   </button>
 </footer>
 </div>