MD Webui: Improve accessibility of toolbar search field

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html

Index: ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html
diff --git a/ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html b/ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html
index c274e197e1c481a08800fbd6babfb6880411a0d0..4b654d763aab5489832d1e6eb9b0f07e91fe93f9 100644
--- a/ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html
+++ b/ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html
@@ -134,10 +134,11 @@
       </paper-spinner-lite>
     </template>
     <paper-icon-button id="icon" icon="cr:search" title="[[label]]"
-        tabindex$="[[computeIconTabIndex_(narrow)]]">
+        tabindex$="[[computeIconTabIndex_(narrow)]]"
+        aria-hidden$="[[computeIconAriaHidden_(narrow)]]">

[dpapad, 2017/03/20 18:07:18]

Is the function call necessary? Can we simply do

aria-hidden$="[[!narrow]]">

[tsergeant, 2017/03/20 23:22:02]

Aria attributes tend to be picky about receiving the full strings
"true"/"false": just giving it the attribute with no value (like
<paper-icon-button aria-hidden>) doesn't seem to trigger the right behavior.

     </paper-icon-button>
     <div id="searchTerm">
-      <label id="prompt" for="searchInput">[[label]]</label>
+      <label id="prompt" for="searchInput" aria-hidden="true">[[label]]</label>
       <input id="searchInput"
           type="search"
           on-input="onSearchTermInput"