Only call X11DesktopHandler for USE_X11 builds

... and the same applies to calling ui::GetCustomFramePrefDefault,
implemented in ui/base/x/x11_util.cc

BUG=295089

Review-Url: https://codereview.chromium.org/2454743002
Cr-Commit-Position: refs/heads/master@{#427830}
2 files changed