Ozone: Avoid blocking in Swapbuffer Call.

ui/ozone/platform/dri/gbm_surfaceless.cc

Index: ui/ozone/platform/dri/gbm_surfaceless.cc
diff --git a/ui/ozone/platform/dri/gbm_surfaceless.cc b/ui/ozone/platform/dri/gbm_surfaceless.cc
index b3748445a369b53d97866b759da0455c26cc4c0d..4a995002cd27ba75dc82a64bf6a60960abeab09a 100644
--- a/ui/ozone/platform/dri/gbm_surfaceless.cc
+++ b/ui/ozone/platform/dri/gbm_surfaceless.cc
@@ -32,8 +32,20 @@ bool GbmSurfaceless::OnSwapBuffers() {
   if (!controller)
     return true;
 
-  bool success = controller->SchedulePageFlip();
+  // Ensure any previous PageFlip events are handled. Doing this here also makes
+  // sure we don't throttle GPU in case it's still handling previous PageFlip
+  // request. This also makes sure we have at most two frames "in flight". Let's
+  // say we submitted all the commands for render buffer A to GPU and then
+  // requested the driver to display it. GPU might still be rendering buffer A,
+  // so the flip is still pending. However, we might already be busy at work
+  // submitting commands for render buffer B. Once done submitting commands for
+  // buffer B, we queue B for page flip, and continue render for frame C. If we
+  // try page flipping B without handling the events, the driver might return
+  // EBusy(in case it's still handling previous flip) here, and also we don't
+  // want to use any more than triple buffering. Thus, we sleep waiting for A
+  // to show up on the screen, before issuing a flip request for B.
   controller->WaitForPageFlipEvent();
+  bool success = controller->SchedulePageFlip();
 
   return success;
 }