Added Base64 support in TextEncoder and TextDecoder.

LayoutTests/fast/encoding/api/base64-expected.txt

+Verify Base64 encoding/decoding
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+decoder = new TextDecoder("base64")
+decoded = decoder.decode(new Uint8Array(bytes))
+PASS encodeURIComponent(string) is encodeURIComponent(decoded)
+encoder = new TextEncoder("base64")
+encoded = encoder.encode(string)
+PASS encoded.length === bytes.length is true
+PASS encoded[i] === bytes[i] is true
+PASS encoded[i] === bytes[i] is true
+PASS encoded[i] === bytes[i] is true
+PASS encoded[i] === bytes[i] is true
+PASS encoded[i] === bytes[i] is true
+PASS encoded[i] === bytes[i] is true
+PASS successfullyParsed is true
+
+TEST COMPLETE
+