Network Log import UI should default to .json files

chrome/browser/resources/net_internals/import_view.html

 <style>
 #import-view-command-line,
 #import-view-user-comments {
   /* Wrap long lines inside of PREs */
   white-space: pre-wrap;
 }
 
 #import-view-load-status-text {
   /* Wrap long lines inside of PREs */
   white-space: pre-wrap;
@@ skipping 17 matching lines @@
 
 .import-view-warning-log {
   color: #e00;
 }
 
 </style>
 
 <div id=import-view-tab-content class=content-box>
   <div>
     <div style='margin-bottom:20px'><b>TIP</b>: Drag-and-drop files into this window.</div>
-    <input type=file value="Load log from file" id=import-view-load-log-file></input>
+    <input type=file value="Load log from file" id=import-view-load-log-file accept=".json"></input>
     <pre id=import-view-load-status-text></pre>
   </div>
 
   <div id=import-view-loaded-div style="display: none">
     <h2>Data Loaded</h2>
     <table class=styled-table>
     <tr>
       <th>Export date</th>
       <td jscontent="timeutil.dateToString(new Date(constants.clientInfo.numericDate))"></td>
     </tr>
@@ skipping 23 matching lines @@
     <tr>
       <th>User comments</th>
       <td>
         <pre id=import-view-user-comments jscontent="userComments"
             jsdisplay="typeof userComments != 'undefined'"></pre>
       </td>
     </tr>
     </table>
   </div>
 </div>