exo: temporarily clamp display scale factor to 2

Version of QT currently in use by Parallels Desktop does not render
correctly when display scale factor is above 2, so let's use 2 as the
upper bound until they switch to a newer version of QT (5.15 should
have the fix).

Bug: b/169984627
Change-Id: Idd31b9a737833fa48ab6b20ae8da5f1bf951b3ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446720
Commit-Queue: Dmitry Torokhov <dtor@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816020}

components/exo/wayland/wayland_display_observer.cc

@@ -117,8 +117,11 @@ bool WaylandDisplayHandler::SendDisplayMetrics(const display::Display& display,
       WL_OUTPUT_SCALE_SINCE_VERSION) {
     // wl_output only supports integer scaling, so if device scale factor is
     // fractional we need to round it up to the closest integer.
-    wl_output_send_scale(output_resource_,
-                         std::ceil(display.device_scale_factor()));
+    // TODO(b/169984627, crbug:1137268): remove clamping to 2 once Parallels
+    // switches to a newer version of QT.
+    wl_output_send_scale(
+        output_resource_,
+        std::min(std::ceil(display.device_scale_factor()), 2.0f));
   }
 
   // TODO(reveman): Send real list of modes.