X11: Set window class for content_shell.

For Chrome, this happens in DesktopBrowserFrameAuraLinux::GetWidgetParams(),
but there was no counterpart for content_shell.

Setting a window class makes it easier to customize one's window manager for
content_shell.

R=peter@chromium.org

Review URL: https://codereview.chromium.org/1880643002

Cr-Commit-Position: refs/heads/master@{#386657}
1 file changed