Reenable optimization on XMLHttpRequest's receive-as-blob functionality

Due to a bug in RedirectToFile functionality of Chromium, the
optimization CL was reverted
(http://src.chromium.org/viewvc/blink?view=revision&revision=168993).

This CL reenables the optimization as
https://codereview.chromium.org/82273002/ fixed the bugs.

BUG=347663

Review URL: https://codereview.chromium.org/311593002

git-svn-id: svn://svn.chromium.org/blink/trunk@175446 bbb929c8-8fbe-4397-9dbb-9b2b20218538