cast_benchmarks: Fix broken behavior
cast_benchmarks: Fix broken behavior

There were two main problems with cast_benchmarks:

1. The initialization tried to use a RunLoop to handle
the closure of initialization of audio and video. Since this
is a fake task runner, the program would just wait forever
given the task runner was not executing tasks.

2. The RTP timestamps on all the frames were fixed! This could
have been a result of modifications to other pieces of code
while cast_benchmarks was unused

This CL fixes both the above behaviors and additionally leverages
existing default config API used by the cast_simulator.

There is a lot of potential cleanup here, but we dont attempt that
in this CL.

BUG=527853
Test=Ran cast_benchmark with --single_run and in full vector space
without any options to completion.

Review URL: https://codereview.chromium.org/1311403006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#347791}

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