[XHR] Reset m_downloadedBlobLength

When XMLHttpRequest.responseType is specified to "blob", we
download the resource into a file and on XMLHttpRequest.response
call, we create a Blob object from m_response.downloadedFilePath()
and m_downloadedBlobLength. m_downloadedBlobLength is accumulated on
WebCore::XMLHttpRequest::didDownloadData() handler call but not
reset on clearResponse() call. So, if you use the XHR instance again
after calling open(), uncleared invalid size will be used.

We should just reset it to 0 on clearResponse() call.

BUG=349321

Review URL: https://codereview.chromium.org/183333005

git-svn-id: svn://svn.chromium.org/blink/trunk@168531 bbb929c8-8fbe-4397-9dbb-9b2b20218538