Fix a broken ASSERT in StylePropertySerializer.
Fix a broken ASSERT in StylePropertySerializer.

We may reach that function asking for a disabled property.
It was too aggressive to put it here.

Review URL: https://chromiumcodereview.appspot.com/14772013

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@149728 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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