Currently, the tracing for background tasks is quite cryptic when
looking at chrome://tracing. Most tasks that are posted appear as
ThreadControllerImpl::RunTask, which when looking at other threads
can be deduced as compile tasks. Improve instrumentation to be more
indicative of the work being done.

BUG:v8:8423

Change-Id: I9b5469154ff7163d743efb1df998ca1df08cd4ce
Reviewed-on: https://chromium-review.googlesource.com/c/1330012
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57413}