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. Use OnceCallback for the 'benchmark done' callback.
. Remove an unncessary callback-runner function.

BUG=none

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: Ib990293e05960755844fd1e5156e459b187c2e1c
Reviewed-on: https://chromium-review.googlesource.com/887163
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532083}