Mac: constrain frame directly inside the move loop

For some reason, when we set the window's frame directly in the move
loop, frame constraints don't trigger right away (as they do during a
window server drag). Instead, the frame goes wherever we told it to,
then constraints kick in, so the window flickers back and forth between
the constrained and unconstrained position.

Since the real fix (window server drag in the move loop) is a ways off,
let's directly constrain inside the move loop.

Bug: 1099600
Change-Id: If71392f90453da868a09ef9f52a4589f9db25c19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276291
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783996}