Fix --force-device-scale-factor on Mac

Previously, Display would read the device scale factor from the command
line in its ctor, but then screen would clobber it.  This meant that
the force device scale factor command line flag was silently ignored.

R=oshima@chromium.org,ccameron@chromium.org

Review URL: https://codereview.chromium.org/1111543003

Cr-Commit-Position: refs/heads/master@{#327211}

ui/gfx/screen_mac.mm

@@ -75,6 +75,10 @@ gfx::Display GetDisplayForScreen(NSScreen* screen) {
     scale = [screen backingScaleFactor];
   else
     scale = [screen userSpaceScaleFactor];
+
+  if (gfx::Display::HasForceDeviceScaleFactor())
+    scale = gfx::Display::GetForcedDeviceScaleFactor();
+
   display.set_device_scale_factor(scale);
   // CGDisplayRotation returns a double. Display::SetRotationAsDegree will
   // handle the unexpected situations were the angle is not a multiple of 90.