fix may be used uninitialized warning on older compilers

Some older compilers give a warning that m_block may be used
uninitialized, since it's not always initialised in the LayoutScope
constructor.

Review URL: https://codereview.chromium.org/170233006

git-svn-id: svn://svn.chromium.org/blink/trunk@167358 bbb929c8-8fbe-4397-9dbb-9b2b20218538