Fix RenderLayer::isUnclippedDescendant().

updateHasUnclippedDescendant() currently gets called for the first time before
updateNeedsCompositedScrolling(), yet it relies on the value of
m_needsCompositedScrolling. This causes the first value of m_isUnclippedDescendant
to be unreliable. This change removes the dependency so that
m_isUnclippedDescendant can be safely computed without knowing
m_needsCompositedScrolling.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23658049

git-svn-id: svn://svn.chromium.org/blink/trunk@157810 bbb929c8-8fbe-4397-9dbb-9b2b20218538