[Net] Add a priority parameter to URLRequest's constructor
This is so that it is clearer what the intended initial priority of
a URLRequest is.
It is also needed so that we can later enforce that if a URLRequest
is set to ignore limits, it has MAXIMUM_PRIORITY; otherwise, we'd have
to mandate that SetPriority() is called before set_load_flags(), which
is fiddly.
Also standardize on a single URLRequest constructor.
BUG=166689
R=jam@chromium.org, jamesr@chromium.org, joth@chromium.org, mmenke@chromium.org, scottbyer@chromium.org, sky@chromium.org, tommi@chromium.org
Review URL: https://codereview.chromium.org/51953002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232219 0039d316-1c4b-4281-b951-d872f2087c98
2ca01e54e2