Add null check for sync layer tree frame sink

Missing a null check that's similar to the one in DemandDrawHw.

Bug: 1104821
Change-Id: I4a1199171edee81220d59f6bc42a06f88df18e95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2305919
Auto-Submit: Bo <boliu@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789932}

third_party/blink/renderer/platform/widget/input/synchronous_compositor_proxy.cc

@@ -138,7 +138,9 @@ void SynchronousCompositorProxy::DemandDrawHw(
 }
 
 void SynchronousCompositorProxy::WillSkipDraw() {
-  layer_tree_frame_sink_->WillSkipDraw();
+  if (layer_tree_frame_sink_) {
+    layer_tree_frame_sink_->WillSkipDraw();
+  }
 }
 
 struct SynchronousCompositorProxy::SharedMemoryWithSize {