Don't check implicit flag for serializing background and -webkit-mask

third_party/WebKit/LayoutTests/fast/css/background-serialize.html

Index: third_party/WebKit/LayoutTests/fast/css/background-serialize.html
diff --git a/third_party/WebKit/LayoutTests/fast/css/background-serialize.html b/third_party/WebKit/LayoutTests/fast/css/background-serialize.html
index 9852529302cb19830eb662bd471d430568e071ac..40b8cf05b51cc6e77e2e356de784d83b08186e00 100644
--- a/third_party/WebKit/LayoutTests/fast/css/background-serialize.html
+++ b/third_party/WebKit/LayoutTests/fast/css/background-serialize.html
@@ -12,7 +12,7 @@ assert_equals(getStyle('.test { background: none; }'), '.test { background: none
 assert_equals(getStyle('.test { background: none; background-color: black;}'), '.test { background: none black; }');
 assert_equals(getStyle('.test { background: none; background-color: initial !important;}'), '.test { background-image: none; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial; background-color: initial !important; }');
 assert_equals(getStyle('.test { background: initial; }'), '.test { background: initial; }');
-assert_equals(getStyle('.test { background: initial; background-color: black; }'), '.test { background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial; background-color: black; }');
+assert_equals(getStyle('.test { background: initial; background-color: black; }'), '.test { background: black; }');
 assert_equals(getStyle('.test { background: inherit; }'), '.test { background: inherit; }');
 assert_equals(getStyle('.test { background: inherit; background-color: black; }'), '.test { background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: black; }');
 assert_equals(getStyle('.test { background: unset; background-color: black; }'), '.test { background-image: unset; background-position: unset; background-size: unset; background-repeat: unset; background-attachment: unset; background-origin: unset; background-clip: unset; background-color: black; }');