Fix uninitialized m_hasOnlySelfCollapsingChildren in RenderBlock

Missing initializer from https://codereview.chromium.org/131223003

Review URL: https://codereview.chromium.org/137313006

git-svn-id: svn://svn.chromium.org/blink/trunk@165092 bbb929c8-8fbe-4397-9dbb-9b2b20218538