[Net] Log RequestPriority instances as strings instead of integers

This is to avoid confusion with whether higher numbers are higher priority.

BUG=307757
R=rch@chromium.org

Review URL: https://codereview.chromium.org/26297006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228814 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228814