Simplify ProgressEvent dispatching methods for readability

This reduces methods exposed on the throttle class and makes
throttling details hidden to XMLHttpRequest class.

- Use dispatchProgressEvent also for loadstart event
- Remove "throttled" from method names
- Move if-clause to switch how to dispatch a ProgressEvent based on its
  name to XMLHttpRequestProgressEventThrottle::dispatchProgressEvent()
- Remove dispatchThrottledProgressEvent and just call
  m_progressEventThrottle.dispatchProgressEvent for each event
- Make XMLHttpRequestProgressEventThrottle::dispatchEvent() private

BUG=none

Review URL: https://codereview.chromium.org/301643002

git-svn-id: svn://svn.chromium.org/blink/trunk@174979 bbb929c8-8fbe-4397-9dbb-9b2b20218538