[XHR] Call dispatchDeferredEvents to simplify the event dispatching.

Because DocumentThreadableLoader doesn't fires events when suspended and
we stop XMLHttpRequestProgressEventThrottle on suspend (1), we can assume that
dispatchReadyStateChangeEvent and dispatchProgressEvent are not called in many
cases. The only exception is the case when they are called after resuming
but before m_dispatchDeferredEventsTimer fires.
On the other hand, the current code works wrongly in such a case because
the actual event dispatching timing doesn't accord with the readystate
change timing (2).

This CL calls dispatchDeferredEvents to fix (2). Calling the method is not
a problem because of (1).
Some code turns dead code because of this change, but I'd keep them
in order to keep this CL simple. I will create another refactoring CL shortly.

BUG=406397

Review URL: https://codereview.chromium.org/496673002

git-svn-id: svn://svn.chromium.org/blink/trunk@180979 bbb929c8-8fbe-4397-9dbb-9b2b20218538