Move delivery of upload 'progress' events to XMLHttpRequestUpload.

Have the XMLHttpRequestUpload object handle dispatching of its
'progress' events, and at the same time keep track of the last
progress data reported.

While the latter state isn't directly made use of here, follow-on
changes to handle 'progress' events during XHR request errors will
depend on having it available.

R=
BUG=315488

Review URL: https://codereview.chromium.org/63033008

git-svn-id: svn://svn.chromium.org/blink/trunk@162254 bbb929c8-8fbe-4397-9dbb-9b2b20218538