[XHR] Move the code to clear variables to internalAbort()

This CL adds internalAbort() call to handleDidCancel(). This is
not harmful. It's rather good that we clear resources.

R=yhirano,sof,kouhei
BUG=406229

Review URL: https://codereview.chromium.org/490083002

git-svn-id: svn://svn.chromium.org/blink/trunk@181791 bbb929c8-8fbe-4397-9dbb-9b2b20218538