Always initialize LayoutScope::m_block.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@167638 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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