MD History: Add spinners when new data is loading or searching

chrome/browser/resources/md_history/history_list.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-item/paper-item.html">
+<link rel="import" href="chrome://resources/polymer/v1_0/paper-spinner/paper-spinner.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_shared_menu/cr_shared_menu.html">
 <link rel="import" href="chrome://history/constants.html">
 <link rel="import" href="chrome://history/history_item.html">
 <link rel="import" href="chrome://history/shared_style.html">
 
 <dom-module id="history-list">
   <template>
     <style include="shared-style"></style>
     <style>
       :host {
         display: flex;
         flex-direction: column;
       }
 
       #infinite-list {
         flex: 1;
         padding-top: var(--first-card-padding-top);
       }
 
+      #loading-spinner {
+        bottom: 15px;
+        left: 200px;

[tsergeant, 2016/04/15 05:14:50]

This 200px is good when the sidebar exists, but when the user is signed out and
there is no sidebar, then the spinner will be off-center again.

One way to solve this is to set `position: relative` on the <history-list> (in
the :host block above).

Then, the #loading-spinner gets positioned relative to the <history-list>
instead of relative to the whole page.

[lshang, 2016/04/18 00:29:00]

Done.
An absolute position element is positioned relative to the first parent element
that has a position other than static. OMG CSS is so mysterious!

[tsergeant, 2016/04/18 01:26:36]

Yup, it's pretty weird sometimes.

+        margin: 0 auto;
+        position: absolute;
+        right: 0;
+        width: 28px;
+      }
+
       paper-item {
         -webkit-user-select: none;
         cursor: pointer;
         font: inherit;
       }
 
       paper-item:hover {
         background: #eaeaea;
       }
     </style>
     <div id="no-results" class="centered-message"
         hidden$="{{hasResults(historyData.length)}}">
       {{noResultsMessage_(searchTerm, loading_)}}
     </div>
     <iron-list items="{{historyData}}" as="item" id="infinite-list"
         on-scroll="scrollHandler_"
         hidden$="{{!hasResults(historyData.length)}}">
       <template>
         <history-item item="[[item]]"
             starred="[[item.starred]]"
             selected="{{item.selected}}"
             is-card-start="[[item.isFirstItem]]"
             is-card-end="[[item.isLastItem]]"
             search-term="[[searchTerm]]"
             number-of-items="[[historyData.length]]">
         </history-item>
       </template>
     </iron-list>
+    <div id="loading-spinner" hidden$="[[!loading_]]">
+      <paper-spinner alt="Loading more history" active></paper-spinner>

[tsergeant, 2016/04/15 05:14:50]

The "Loading more history" string needs to be localized.

I think it would be fine to reuse the "Loading..." string from history.html,
which can be accessed with `$i18n{loading}`

[lshang, 2016/04/18 00:28:59]

Done.

+    </div>
     <cr-shared-menu id="sharedMenu">
       <paper-item id="menuMoreButton" on-tap="onMoreFromSiteTap_">
         $i18n{moreFromSite}
       </paper-item>
       <paper-item id="menuRemoveButton" on-tap="onRemoveFromHistoryTap_">
         $i18n{removeFromHistory}
       </paper-item>
     </cr-shared-menu>
   </template>
   <script src="chrome://history/history_list.js"></script>
 </dom-module>