Fix nullptr access in SurfaceLayer::SetFallbackSurfaceId

Bug: 868828,827242
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I13199db914192ab81bbe6fd34b086b58fa275fa4
Reviewed-on: https://chromium-review.googlesource.com/1155005Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Commit-Queue: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579114}

cc/layers/surface_layer.cc

@@ -72,7 +72,8 @@ void SurfaceLayer::SetFallbackSurfaceId(const viz::SurfaceId& surface_id) {
     // TODO(samans): This was added to fix https://crbug.com/827242. Remove this
     // once fallback SurfaceIds aren't tied to SurfaceReferences, and
     // viz::Display can handle missing fallbacks. https://crbug.com/857575
-    layer_tree_host()->SetNeedsCommitWithForcedRedraw();
+    if (layer_tree_host())
+      layer_tree_host()->SetNeedsCommitWithForcedRedraw();
     return;
   }