Do not normalize into NFC the data sent by XMLHttpRequest

This is a follow-up to http://crbug.com/117128, the patch for which
(https://src.chromium.org/viewvc/blink?view=rev&revision=154881)
introduced the TextEncoding::normalizeAndEncode API method for when
normalization into Unicode NFC is desired in addition to encoding. If
normalization is not desired, then the TextEncoding::encode API can be
used instead.

As reported by David Benjamin, XHR request data was being normalized.
This causes issues when the data sent is in JSON format, and a string
literal has a JSON newline escape sequence "\n" followed by a Unicode
combining char. Before this change, the combining char was combined
with the 'n', leading to invalid JSON like "\ǹ".

The following browsers were tested and found not to normalize the XHR
request data:
- IE 7
- IE 8
- IE 9
- IE 10
- IE 11 Preview
- Firefox 3.6.28
- Firefox 23.0.1
- Opera 12.16

Three new tests are added:
- request-encoding3.html: Test that passing a string to the send() method
  does not result in the string being normalized.
- request-encoding4.html: Test that passing a document to the send() method
  does not result in the serialized data being normalized.
- response-encoding2.html: Test that XHR response text is not normalized.

BUG=277526

Review URL: https://chromiumcodereview.appspot.com/22875053

git-svn-id: svn://svn.chromium.org/blink/trunk@156678 bbb929c8-8fbe-4397-9dbb-9b2b20218538