Refactor XMLHttpRequestProgressEventThrottle for readability

- Revise comments to distinguish events named "progress" and events
  using the ProgressEvent interface.
- Rename deliverProgressEvent() to make it clear it's for scheduled one.
- Add one ASSERT
- Add FIXME about dispatchReadyStateChangeEvent()'s progress event
  coalescing
- Explained terminology in the class comment
- Use XMLHttpRequestProgressEvent for m_deferredProgressEvent

BUG=none
R=yhirano,sof

Review URL: https://codereview.chromium.org/496443002

git-svn-id: svn://svn.chromium.org/blink/trunk@180760 bbb929c8-8fbe-4397-9dbb-9b2b20218538