Make sure client exists before dispatching font invalidation

Bug: 988125
Change-Id: I989a74f5b30ccf9d9d4b5d3d00fdd56131dd5df5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1801226
Auto-Submit: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697762}

third_party/blink/renderer/core/css/css_font_selector.cc

@@ -76,8 +76,11 @@ void CSSFontSelector::DispatchInvalidationCallbacks() {
 
   HeapVector<Member<FontSelectorClient>> clients;
   CopyToVector(clients_, clients);
-  for (auto& client : clients)
-    client->FontsNeedUpdate(this);
+  for (auto& client : clients) {
+    if (client) {
+      client->FontsNeedUpdate(this);
+    }
+  }
 }
 
 void CSSFontSelector::FontFaceInvalidated() {