Added Base64 support in TextEncoder and TextDecoder.

LayoutTests/fast/encoding/api/resources/shared.js

 // Only these encodings are supported for encoding (vs. decoding)
 utf_encodings = ["utf-8", "utf-16le", "utf-16be"];
 
 // From non-normative encodings.json resource referenced by http://encoding.spec.whatwg.org/
 encodings_table = [
     {
         "encodings": [
             {
                 "labels": [
                     "unicode-1-1-utf-8",
@@ skipping 445 matching lines @@
                 "name": "utf-16le"
             },
             {
                 "labels": [
                     "x-user-defined"
                 ],
                 "name": "x-user-defined"
             }
         ],
         "heading": "Legacy miscellaneous encodings"
+    },
+    {
+        "encodings": [
+            {
+                "labels": [
+                    "base64",
+                ],
+                "name": "base64"
+            },
+        ],
+        "heading": "Base64 encodings"
     }
 ];