Add missing null check in OffscreenCanvas::Commit

BUG=866241

Change-Id: Ic789573da753fe1e438778d777556a659fbf0ba9
Reviewed-on: https://chromium-review.googlesource.com/1147460Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577322}

third_party/blink/renderer/core/offscreencanvas/offscreen_canvas.cc

@@ -45,7 +45,7 @@ OffscreenCanvas::~OffscreenCanvas() = default;
 
 void OffscreenCanvas::Commit(scoped_refptr<CanvasResource> canvas_resource,
                              const SkIRect& damage_rect) {
-  if (!HasPlaceholderCanvas())
+  if (!HasPlaceholderCanvas() || !canvas_resource)
     return;
 
   base::TimeTicks commit_start_time = WTF::CurrentTimeTicks();