Add logs to XMLHttpRequest to ease debugging.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@159234 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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