Prevent data race from low-pass filter.

Makes main thread wait for the filter level to be picked to avoid a race
between the LPF thread and update_reference_frames(). This also
re-enables the failing tests under thread_sanitizer where this data race
was detected.

BUG=webm:1460

Change-Id: I7f5797142ea0200394309842ce3e91a480be4fbc