Clear non-inherited properties on anonymous block

Add a modify_style_for_anonymous_flow() function to use initial values
for non-inherited properties and parent values for inherited properties
as the block style. It also set border and outline to zero and set the
display property from the parameter.

tests/wpt/metadata-css/css-flexbox-1_dev/html/flexbox_generated-flex.htm.ini

@@ -1,3 +0,0 @@
-[flexbox_generated-flex.htm]
-  type: reftest
-  expected: FAIL