Bug 314190

Currently there are some locations in the code that do NOT check whether wpe_screen_get_refresh_rate() returns -1, which is a sentinel value used to indicate that the platform implementation has not provided a refresh rate value. I could find two call sites without checks, for example in DisplayVBlankMonitorWPE.cpp: return makeUnique<DisplayVBlankMonitorWPE>(wpe_screen_get_refresh_rate(screen) / 1000, WTF::move(observer)); and in WPEViewDRM.cpp we can end up with a negative value for priv->refreshDuration: priv->refreshDuration = Seconds(1 / (wpe_screen_get_refresh_rate(wpeDisplayDRMGetScreen(display)) / 1000.));