Add tooltip description for settings search page

Use the tooltip description in the aria label so it's spoken by
ChromeVox.

Before video: http://crbug/1111569#c9
After video: http://crbug/1111569#c10


Fixed: 1111569
Change-Id: I637281ce9b0727f05bed0809ece6f29819bb79d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462375Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815829}

chrome/browser/resources/settings/chromeos/os_search_page/os_search_page.html

@@ -68,7 +68,8 @@
               </cr-policy-pref-indicator>
             </template>
             <select class="md-select" on-change="onChange_"
-                aria-labelledby="searchExplanation"
+                aria-label="$i18n{osSearchEngineLabel}
+                    $i18n{osSearchEngineTooltip}"
                 disabled$="[[isDefaultSearchEngineEnforced_(
                     prefs.default_search_provider_data.template_url_data)]]"
                 deep-link-focus-id$="[[Setting.kPreferredSearchEngine]]">