Fix case-sensitivity problems in extension content verification

extensions/test/data/content_verifier/payload.json

 {
   "content_hashes": [
     {
       "block_size": 4096,
       "hash_block_size": 4096,
       "format": "treehash",
       "files": [
         {
           "path": "manifest.json",
           "root_hash": "-vyyIIn7iSCzg7X3ICUI5wZa3tG7w7vyiCckxZdJGfs"
         },
         {
           "path": "background.js",
           "root_hash": "txHiG5KQvNoPOSH5FbQo9Zb5gJ23j3oFB0Ru9DOnziw"
         },
         {
           "path": "foo/bar.html",
           "root_hash": "L37LFbT_hmtxRL7AfGZN9YTpW6yoz_ZiQ1opLJn1NZU"
+        },
+        {
+          "path": "lowercase.html",
+          "root_hash": "HpLotLGCmmOdKYvGQmD3OkXMKGs458dbanY4WcfAZI0"
+        },
+        {
+          "path": "ALLCAPS.HTML",
+          "root_hash": "bl-eV8ENowvtw6P14D4X1EP0mlcMoG-_aOx5o9C1364"
+        },
+        {
+          "path": "MixedCase.Html",
+          "root_hash": "zEAO9FwciigMNy3NtU2XNb-dS5TQMmVNx0T9h7WvXbQ"
+        },
+        {
+          "path": "mIxedcAse.Html",
+          "root_hash": "nKRqUcJg1_QZWAeCb4uFd5ouC0McuGavKp8TFDRqBgg"
         }
       ]
     }
   ],
   "item_id": "abcdefghijklmnopabcdefghijklmnop",
   "item_version": "1.2.3"
 }