Mark variable-declaration-18.html as flaky ...like it's neighbors -16 and -17
Mark variable-declaration-18.html as flaky
...like it's neighbors -16 and -17

TBR=kojii@chromium.org
NOTRY=true
BUG=609760, 582836

Review-Url: https://codereview.chromium.org/1953283002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392052}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk