Add refresh_rate_millihertz for MonitorHandle

This also alters `VideoMode::refresh_rate` to
`VideoMode::refresh_rate_millihertz` which now returns monitor refresh rate in
mHz.

src/platform_impl/web/monitor.rs

@@ -17,6 +17,10 @@ impl MonitorHandle {
         None
     }
 
+    pub fn refresh_rate_millihertz(&self) -> Option<u32> {
+        None
+    }
+
     pub fn size(&self) -> PhysicalSize<u32> {
         PhysicalSize {
             width: 0,
@@ -41,8 +45,8 @@ impl VideoMode {
         unimplemented!();
     }
 
-    pub fn refresh_rate(&self) -> u16 {
-        32
+    pub fn refresh_rate_millihertz(&self) -> u32 {
+        32000
     }
 
     pub fn monitor(&self) -> RootMonitorHandle {