[wpt] Don't make unimplemented CSS properties PASS.

For computed style and inheritance tests, we silently passed
unimplemented CSS properties. Assert that they exist, and if not, fail
instead.

R=ericwilligers@chromium.org

Change-Id: I1f62bb2ef93b84528373b329dadefb6162ce19c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609850
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660172}

css/support/computed-testcommon.js

@@ -14,8 +14,7 @@ function test_computed_value(property, specified, computed) {
     computed = specified;
   test(() => {
     const target = document.getElementById('target');
-    if (!getComputedStyle(target)[property])
-      return;
+    assert_own_property(getComputedStyle(target), property);
     target.style[property] = '';
     target.style[property] = specified;
     assert_equals(getComputedStyle(target)[property], computed);

css/support/inheritance-testcommon.js

@@ -5,8 +5,7 @@
 function assert_initial(property, initial) {
   test(() => {
     const target = document.getElementById('target');
-    if (!getComputedStyle(target)[property])
-      return;
+    assert_own_property(getComputedStyle(target), property);
     target.style[property] = 'initial';
     assert_equals(getComputedStyle(target)[property], initial);
     target.style[property] = '';
@@ -29,8 +28,7 @@ function assert_inherited(property, initial, other) {
   test(() => {
     const container = document.getElementById('container');
     const target = document.getElementById('target');
-    if (!getComputedStyle(target)[property])
-      return;
+    assert_own_property(getComputedStyle(target), property);
     container.style[property] = 'initial';
     target.style[property] = 'unset';
     assert_not_equals(getComputedStyle(container)[property], other);
@@ -65,8 +63,7 @@ function assert_not_inherited(property, initial, other) {
   test(() => {
     const container = document.getElementById('container');
     const target = document.getElementById('target');
-    if (!getComputedStyle(target)[property])
-      return;
+    assert_own_property(getComputedStyle(target), property);
     container.style[property] = 'initial';
     target.style[property] = 'unset';
     assert_not_equals(getComputedStyle(container)[property], other);