MutationObserver: Fix a null-pointer dereference in MutationObserverRegistration::unregister.

third_party/WebKit/Source/core/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/chromecast_build.gni")
 import("//build/toolchain/toolchain.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//third_party/WebKit/Source/bindings/bindings.gni")
@@ skipping 1095 matching lines @@
     "dom/DOMImplementationTest.cpp",
     "dom/DOMMatrixTest.cpp",
     "dom/DocumentStatisticsCollectorTest.cpp",
     "dom/DocumentTest.cpp",
     "dom/DocumentUserGestureTokenTest.cpp",
     "dom/ElementTest.cpp",
     "dom/ElementVisibilityObserverTest.cpp",
     "dom/ExecutionContextTaskTest.cpp",
     "dom/IdleDeadlineTest.cpp",
     "dom/MainThreadTaskRunnerTest.cpp",
+    "dom/MutationObserverTest.cpp",
     "dom/NodeTest.cpp",
     "dom/NthIndexCacheTest.cpp",
     "dom/RangeTest.cpp",
     "dom/ScriptRunnerTest.cpp",
     "dom/SelectorQueryTest.cpp",
     "dom/StaticRangeTest.cpp",
     "dom/StyleElementTest.cpp",
     "dom/StyleEngineTest.cpp",
     "dom/TextTest.cpp",
     "dom/TreeScopeStyleSheetCollectionTest.cpp",
@@ skipping 270 matching lines @@
   sources = [
     "frame/csp/ContentSecurityPolicyFuzzer.cpp",
   ]
   deps = [
     ":core",
     "//third_party/WebKit/Source/platform:blink_fuzzer_test_support",
   ]
   dict = "//testing/libfuzzer/fuzzers/dicts/content_security_policy.dict"
   seed_corpus = "//testing/libfuzzer/fuzzers/content_security_policy_corpus"
 }