viz: Add null check for |damage_tracker_|

This check fixes the broken perf test: RemoveOverdrawQuadPerfTest.*

Test: viz_perftests
Change-Id: I4afcf5234feb6d810f066f3f5cb9eae7d2559740
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2047419
Commit-Queue: Sasha McIntosh <sashamcintosh@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740387}

components/viz/service/display/display.cc

@@ -227,7 +227,8 @@ Display::~Display() {
   if (scheduler_)
     scheduler_->SetClient(nullptr);
 
-  damage_tracker_->RunDrawCallbacks();
+  if (damage_tracker_)
+    damage_tracker_->RunDrawCallbacks();
 }
 
 void Display::Initialize(DisplayClient* client,