CFI: fix invalid cast in tile_manager_unittest.cc.
CFI: fix invalid cast in tile_manager_unittest.cc.

TaskSetFinishedTaskImpl was casted to its sibling,
cc::RasterTask instead of a base class, cc::TileTask.

This is a follow up to https://codereview.chromium.org/1470113002 which introduced this bad cast.

BUG=565515,464797
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1494273002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#363063}

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