Imply m_needsToRecomputeCompositingRequirements from RenderLayerCompositor's updateType

We eventually want to remove m_needsToRecomputeCompositingRequirements and
drive the system based on RenderLayerCompositor's updateType. This CL removes
one place where we set this dirty bit explicitly.

Review URL: https://codereview.chromium.org/229943004

git-svn-id: svn://svn.chromium.org/blink/trunk@171153 bbb929c8-8fbe-4397-9dbb-9b2b20218538