Fix an uninitialized read in shouldBeSelfPaintingLayer

https://codereview.chromium.org/125043002 allowed the code
to read m_isOverflowOnlyLayer in the RenderLayer constructor
before it is set. The uninitialized read should not yield to
any bad behavior as we always call the style change logic on
a new layer, which recomputes m_isSelfPaintingLayer, after
setting m_isOverflowOnlyLayer.

BUG=333564

Review URL: https://codereview.chromium.org/137393006

git-svn-id: svn://svn.chromium.org/blink/trunk@165165 bbb929c8-8fbe-4397-9dbb-9b2b20218538