Bug 1320320 - remove the missed-epoch-checking panic in wait_for_epoch() WR bindings. r=mchang?

If render_backend hasn't process any display_list yet, there will no epoch data in renderer. So, current_epoch() call will return None in that case.

MozReview-Commit-ID: CDJ2UJ8AHhT

gfx/webrender_bindings/src/bindings.rs

@@ -456,9 +456,6 @@ fn wait_for_epoch(window: &mut WrWindowState) {
                     continue 'outer;
                 }
             }
-            else {
-                panic!("Could not get an epoch from the renderer");
-            }
 
             // If the epoch is not matched, starts to wait for next frame updating.
             while !*finished {