Reset the load priority when converting an async request to sync.

This bug caused http://codereview.chromium.org/51683002 to be reverted, which will ensure it is tested when it re-lands.

BUG=none

Review URL: https://codereview.chromium.org/60613002

git-svn-id: svn://svn.chromium.org/blink/trunk@161381 bbb929c8-8fbe-4397-9dbb-9b2b20218538