DEPS.xwalk: Roll blink-crosswalk (6f18c5b -> e8b6b99).

e8b6b99 Merge pull request #12 from rakuco/backport-blink-r167638
901e77a [Backport r167638] Always initialize LayoutScope::m_block.
135bf8e Merge pull request #11 from rakuco/backport-blink-r167358
419801e [Backport r167358] fix may be used uninitialized warning on older
        compilers

This should fix the Tizen Generic build after we moved chromium-crosswalk to
34.0.1847.45.