Allow nullptr in Style[Non]InheritedVariables::SetVariable.

This does not currently cause a crash, entirely because of luck. A value
of nullptr is passed to SetVariable whenever an invalid var()-reference
is used (for example), but it just so happens that needs_resolution_ is
always true in this case, so it short circuits before attempting to de-
reference 'value'.

R=futhark@chromium.org

Bug: 641877
Change-Id: I8b3776a58efb82b5deef74b50561d2ebfd57b817
Reviewed-on: https://chromium-review.googlesource.com/1235725
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593136}

third_party/blink/renderer/core/css/resolver/css_variable_resolver_test.cc

@@ -362,4 +362,20 @@ TEST_F(CSSVariableResolverTest, RemoveNonInheritedVariable) {
   EXPECT_FALSE(non_inherited_variables->RegisteredVariable(name));
 }
 
+TEST_F(CSSVariableResolverTest, DontCrashWhenSettingInheritedNullVariable) {
+  scoped_refptr<StyleInheritedVariables> inherited_variables =
+      StyleInheritedVariables::Create();
+  AtomicString name("--test");
+  inherited_variables->SetVariable(name, nullptr);
+  inherited_variables->SetRegisteredVariable(name, nullptr);
+}
+
+TEST_F(CSSVariableResolverTest, DontCrashWhenSettingNonInheritedNullVariable) {
+  std::unique_ptr<StyleNonInheritedVariables> inherited_variables =
+      StyleNonInheritedVariables::Create();
+  AtomicString name("--test");
+  inherited_variables->SetVariable(name, nullptr);
+  inherited_variables->SetRegisteredVariable(name, nullptr);
+}
+
 }  // namespace blink

third_party/blink/renderer/core/style/style_inherited_variables.h

@@ -33,8 +33,9 @@ class CORE_EXPORT StyleInheritedVariables
 
   void SetVariable(const AtomicString& name,
                    scoped_refptr<CSSVariableData> value) {
-    needs_resolution_ = needs_resolution_ || value->NeedsVariableResolution() ||
-                        value->NeedsUrlResolution();
+    needs_resolution_ =
+        needs_resolution_ || (value && (value->NeedsVariableResolution() ||
+                                        value->NeedsUrlResolution()));
     data_.Set(name, std::move(value));
   }
   CSSVariableData* GetVariable(const AtomicString& name) const;

third_party/blink/renderer/core/style/style_non_inherited_variables.h

@@ -35,8 +35,9 @@ class CORE_EXPORT StyleNonInheritedVariables {
 
   void SetVariable(const AtomicString& name,
                    scoped_refptr<CSSVariableData> value) {
-    needs_resolution_ = needs_resolution_ || value->NeedsVariableResolution() ||
-                        value->NeedsUrlResolution();
+    needs_resolution_ =
+        needs_resolution_ || (value && (value->NeedsVariableResolution() ||
+                                        value->NeedsUrlResolution()));
     data_.Set(name, std::move(value));
   }
   CSSVariableData* GetVariable(const AtomicString& name) const;