When we run TurboFan compilation for WebAssembly on worker threads with
default priority, we see in bug reports (https://crbug.com/914757) and
in experiments that TurboFan compilation can block other, higher
priority tasks. Therefore we want to post TurboFan compilation tasks
with lower priority.

A quick prototype showed that if we run all WebAssembly compilation with
low priority, the problem described in https://crbug.com/914757 is
fixed.

R=adamk@chromium.org
CC=rmcilroy@chromium.org

Bug: chromium:920181
Change-Id: I85e2c0c6a96ff9ef165a23ef8eb531944b20f2b0
Reviewed-on: https://chromium-review.googlesource.com/c/1402790Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58792}