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

git-svn-id: svn://svn.chromium.org/blink/trunk@167638 bbb929c8-8fbe-4397-9dbb-9b2b20218538