Ensure we never remove the style attribute when syncing it from CSSOM.

third_party/WebKit/LayoutTests/custom-elements/spec/callback.html

Index: third_party/WebKit/LayoutTests/custom-elements/spec/callback.html
diff --git a/third_party/WebKit/LayoutTests/custom-elements/spec/callback.html b/third_party/WebKit/LayoutTests/custom-elements/spec/callback.html
index e0c297b4ab59fcff7e8c6f6097fb2f3dfe5588da..cec28ba7ac0f094e8c7ff18fc806b123f7f6f7ee 100644
--- a/third_party/WebKit/LayoutTests/custom-elements/spec/callback.html
+++ b/third_party/WebKit/LayoutTests/custom-elements/spec/callback.html
@@ -105,7 +105,7 @@
 
     element.style.color = '';
     assert_equals(logs.length, 3);
-    assert_log_is_type(logs, 2, attributeChanged, element, ['style', 'color: green;', null, null]);
+    assert_log_is_type(logs, 2, attributeChanged, element, ['style', 'color: green;', "", null]);
   }, 'style.color should enqueue attributeChangedCallback for style attribute');
 
   test_with_window(w => {
@@ -133,7 +133,7 @@
 
     element.style.removeProperty('color', 'red');
     assert_equals(logs.length, 2);
-    assert_log_is_type(logs, 1, attributeChanged, element, ['style', 'color: red;', null, null]);
+    assert_log_is_type(logs, 1, attributeChanged, element, ['style', 'color: red;', "", null]);
   }, 'style.setProperty/removeProperty should enqueue attributeChangedCallback for style attribute');
 
   test_with_window(w => {