[ozone] Fixed the logic that forces using the custom frame.

This CL fixes the linked crbug by improving the condition checked in
DesktopBrowserFrameAuraLinux::UseCustomFrame() when Ozone is enabled.

The method returns whether the window frame should use client-side
decorations, and it returned true by mistake.  The wrong condition
assumed that if the platform suggests using the custom frame (that is,
OzonePlatform::ShouldUseCustomFrame() returns true), that means that the
platform lacks support for server-side decorations.  In fact, that is
not correct; the right thing to check is the runtime platform property
named supports_server_side_window_decorations [1] that is not a
suggestion but indicates clearly whether the feature is supported.

[1] https://source.chromium.org/chromium/chromium/src/+/master:ui/ozone/public/ozone_platform.h;l=162;bpv=1;bpt=0;drc=66b50ba4201acf8356be5419eee5404d03d52d7a

Bug: 1243937
Change-Id: Iaf69f6e6ef2f178dc568f5c30847dcce42f3d896
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3133066
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/main@{#918438}