Replace error-prone value with check from CSSProperty

In EditingStyle, there is a static integer that specifies the number of
non-inherited editing properties, that needed to appear in the beginning
of editingProperties vector. A better approach is to have a runtime
check using CSSProperty::isInheritedProperty().

BUG=269140

Review URL: https://chromiumcodereview.appspot.com/22378004

git-svn-id: svn://svn.chromium.org/blink/trunk@155788 bbb929c8-8fbe-4397-9dbb-9b2b20218538