Fix incorrect comment about XMLHttpRequest::handleDidCancel()
Fix incorrect comment about XMLHttpRequest::handleDidCancel()

handleDidCancel() doesn't handle m_loader->cancel() in internalAbort().
It's for handling cancellation coming from other components.

BUG=none

Review URL: https://codereview.chromium.org/467593002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@180078 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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