Ensure we don't crash when asking for variables that don't exist from the computed style.

third_party/WebKit/LayoutTests/fast/css/variables/read-from-computed-style.html

Index: third_party/WebKit/LayoutTests/fast/css/variables/read-from-computed-style.html
diff --git a/third_party/WebKit/LayoutTests/fast/css/variables/read-from-computed-style.html b/third_party/WebKit/LayoutTests/fast/css/variables/read-from-computed-style.html
index f0d2ea26687fa165db197fe818ab3b4754aeb375..730033be50dfea462d9a0f4d73391e3845a5fb54 100644
--- a/third_party/WebKit/LayoutTests/fast/css/variables/read-from-computed-style.html
+++ b/third_party/WebKit/LayoutTests/fast/css/variables/read-from-computed-style.html
@@ -23,23 +23,27 @@ test(function() {
   assert_equals(getComputedStyle(container).getPropertyValue("--a"), " it was the best of times");
   assert_equals(getComputedStyle(container).getPropertyValue("--b"), " it was the worst of times");
   assert_equals(getComputedStyle(container).getPropertyValue("--c"), " also the blurst of times");
+  assert_equals(getComputedStyle(container).getPropertyValue("--d"), "");
 }, 'Directly declared custom properties appear in computed style.');
 
 test(function() {
   assert_equals(getComputedStyle(a).getPropertyValue("--a"), " it was the best of times");
   assert_equals(getComputedStyle(a).getPropertyValue("--b"), " it was the worst of times");
   assert_equals(getComputedStyle(a).getPropertyValue("--c"), " also the blurst of times");
+  assert_equals(getComputedStyle(a).getPropertyValue("--d"), "");
 }, 'Inherited custom properties appear in computed style.');
 
 test(function() {
   assert_equals(getComputedStyle(b).getPropertyValue("--a"), " it was the best of times");
   assert_equals(getComputedStyle(b).getPropertyValue("--b"), " it was the worst of times");
   assert_equals(getComputedStyle(b).getPropertyValue("--c"), " it was the age of wisdom");
+  assert_equals(getComputedStyle(b).getPropertyValue("--d"), "");
 }, 'Inherited custom properties overidden values appear correctly in computed style.');
 
 test(function() {
   assert_equals(getComputedStyle(c).getPropertyValue("--a"), " it was the best of times");
   assert_equals(getComputedStyle(c).getPropertyValue("--b"), " it was the worst of times");
   assert_equals(getComputedStyle(c).getPropertyValue("--c"), " it was the age of wisdom");
+  assert_equals(getComputedStyle(c).getPropertyValue("--d"), "");
 }, 'Overidden and inherited custom properties appear in computed style.');
 </script>