[Backport r167638] Always initialize LayoutScope::m_block.

While the missing initialization was completely harmless,
  I got a compilation error with gcc 4.7.2.

  BUG=none

  Review URL: https://codereview.chromium.org/176433002

Author: mstensho@opera.com

This is another change needed to fix the build with Tizen Generic. Verified to
be the last one.