Reorganize clearing methods in XMLHttpRequest

m_receivedLength, m_decoder and m_responseEncoding are not used once loading
is finished. Group them and reset in a method named clearVariablesForLoading.

Add some comments about response holder variables.

Some code reorg for readability in didReceiveData().

BUG=

Review URL: https://codereview.chromium.org/25472002

git-svn-id: svn://svn.chromium.org/blink/trunk@158623 bbb929c8-8fbe-4397-9dbb-9b2b20218538