Remove SerializedScriptValue special case from ShouldKeepAttributeAlive

Just noticed that this case is dead code;
there's only one readonly SerializedScriptValue attribute, but its getter
is custom, so this isn't relevant.

Source/core/frame/History.idl:
[Custom=Getter] readonly attribute SerializedScriptValue state;

R=haraken

Review URL: https://codereview.chromium.org/131053002

git-svn-id: svn://svn.chromium.org/blink/trunk@164754 bbb929c8-8fbe-4397-9dbb-9b2b20218538