[Backport r167358] 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

Author: mostynb@opera.com

This should fix the build problems in Tizen Generic:
https://build.crosswalk-project.org/builders/Crosswalk%20Tizen%20Generic%2064-bits/builds/38/steps/compile/logs/stdio/text