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SurfaceDependencies is a type alias that was introduced in PendingFrameObserver
but is not used consistently making understanding types more difficult. This
CL simplify removes the type alias.

BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2861713003
Cr-Commit-Position: refs/heads/master@{#469476}