Don't use the window.open path for most user-initiated new window creations.

Currently, if the user requests a new window with key modifiers, we don't give
the embedder a chance to move the load to a new process, we just immediately
open it in the current process.

We do need to guarantee same-process new windows for form submissions with
with click modifiers, because we switch the method to GET during the process
transition.

BUG=331321
TEST=WebFrameTest.ModifiedClickNewWindow

Review URL: https://codereview.chromium.org/170703003

git-svn-id: svn://svn.chromium.org/blink/trunk@167721 bbb929c8-8fbe-4397-9dbb-9b2b20218538