Fix segfault in AtRuleCSSStyleDeclaration::getPropertyValue

Bug: 803453, 803520
Change-Id: I3870ea56cd8349704621f3a72b64b69a24624680
Reviewed-on: https://chromium-review.googlesource.com/874981Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: meade_UTC10 <meade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530434}

third_party/WebKit/LayoutTests/fast/css/invalid-font-face-crash.html

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<style id="styleElement">
+@font-face {}
+</style>
+<script>
+test(() => {
+  let fontFaceRule = styleElement.sheet.rules[0];
+  assert_equals(fontFaceRule.style.getPropertyValue('font-display'), "");
+}, "This test should not crash");
+</script>

third_party/WebKit/Source/core/css/AtRuleCSSStyleDeclaration.cpp

@@ -51,7 +51,11 @@ String AtRuleCSSStyleDeclaration::getPropertyValue(
   if (id == AtRuleDescriptorID::Invalid)
     return g_empty_string;
 
-  return descriptor_value_set_->GetPropertyCSSValue(id)->CssText();
+  const CSSValue* value = descriptor_value_set_->GetPropertyCSSValue(id);
+  if (value)
+    return value->CssText();
+
+  return g_empty_string;
 }
 
 String AtRuleCSSStyleDeclaration::getPropertyPriority(