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As described in
https://docs.google.com/document/d/1va_h2uei_-H-AjXMSxdSOat2vkwD4FKREn_CK-wRZes/edit?usp=sharing,
clearing the cache in Lighthouse can be slow.

This CL fixes the problem by increasing the priority of the
WebRtcEventLogManager TaskRunner to USER_BLOCKING when the cache is
being cleared. This is the correct priority for an operation on which
the user is waiting.

Change-Id: If5aea24d2388dd67ecbaead8255886905202daba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1882029Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710323}