Add logs to XMLHttpRequest to ease debugging.

In debugging complicated XHR bugs, it's important to understand sequence
of method calls and state transition. I want to upstream simple logs at the
head of each method implementations and loader delegate methods I'm regularly
putting for debugging.

BUG=

Review URL: https://codereview.chromium.org/26231002

git-svn-id: svn://svn.chromium.org/blink/trunk@159234 bbb929c8-8fbe-4397-9dbb-9b2b20218538