chromeos: Fix "open appearance settings" icon in OS a11y settings

When SplitSettings is enabled (the default) the browser Appearance
section opens in a tab. In this case, use the "external" link out
icon.

https://screenshot.googleplex.com/47LQzbfZeTx

If SplitSettings is disabled, keep the normal in-page link icon.

Bug: 999354
Change-Id: I7f70ac3a18c4a92b7d24a5b0be926488ec0010b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796784
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695369}

chrome/browser/resources/settings/a11y_page/manage_a11y_page.html

@@ -110,7 +110,8 @@
         embedded></cr-link-row>
     <cr-link-row class="hr" label="$i18n{appearanceSettingsTitle}"
         on-click="onAppearanceTap_"
-        sub-label="$i18n{appearanceSettingsDescription}" embedded></cr-link-row>
+        sub-label="$i18n{appearanceSettingsDescription}"
+        external="[[isOSSettings_]]" embedded></cr-link-row>
 
     <h2>$i18n{keyboardAndTextInputHeading}</h2>
     <settings-toggle-button class="first"

chrome/browser/resources/settings/a11y_page/manage_a11y_page.js

@@ -125,6 +125,18 @@ Polymer({
       }
     },
 
+    /**
+     * Whether this page shown as part of OS settings.
+     * TODO(crbug.com/986596): Remove this when SplitSettings is the default.
+     * @private
+     */
+    isOSSettings_: {
+      type: Boolean,
+      value: function() {
+        return loadTimeData.getBoolean('isOSSettings');
+      },
+    },
+
     /**
      * |hasKeyboard_|starts undefined so observers don't trigger
      * until it has been populated.