Assert exceptionCode is non zero in XMLHttpRequest::handleRequestError()
Assert exceptionCode is non zero in XMLHttpRequest::handleRequestError()

Currently, the if condition includes exceptionCode. I.e. even if
m_async is false, we proceed to dispatch events when exceptionCode is
zero. This never happens as we don't call this function with m_async
set to false and exceptionCode set to 0.

To make it clear that we never hit this case, remove it from the
condition and have an ASSERT instead.

BUG=none
R=yhirano

Review URL: https://codereview.chromium.org/1227343003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199801 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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