Fix segfault in FlingScheduler

Don't call into |fling_controller_| if it's destroyed.

Bug: 852264
Change-Id: I71081588f0b7e5bf882c28231e3d6b20026e4611
Reviewed-on: https://chromium-review.googlesource.com/1099145Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567258}

content/browser/renderer_host/input/fling_scheduler.cc

@@ -78,7 +78,9 @@ ui::Compositor* FlingScheduler::GetCompositor() {
 }
 
 void FlingScheduler::OnAnimationStep(base::TimeTicks timestamp) {
-  fling_controller_->ProgressFling(timestamp);
+  DCHECK(observed_compositor_);
+  if (fling_controller_)
+    fling_controller_->ProgressFling(timestamp);
 }
 
 void FlingScheduler::OnCompositingShuttingDown(ui::Compositor* compositor) {

content/browser/renderer_host/input/fling_scheduler_android.cc

@@ -66,7 +66,9 @@ ui::WindowAndroid* FlingSchedulerAndroid::GetRootWindow() {
 }
 
 void FlingSchedulerAndroid::OnAnimate(base::TimeTicks frame_begin_time) {
-  fling_controller_->ProgressFling(frame_begin_time);
+  DCHECK(observed_window_);
+  if (fling_controller_)
+    fling_controller_->ProgressFling(frame_begin_time);
 }
 
 }  // namespace content