This introduces a new {base::SharedMutex}, which mimics
{std::shared_mutex}, available in C++17.
It is being used for synchronizing the WebAssembly background compile
tasks. This removes a lot of unnecessary contention, leading to
synchronization of background tasks that should be able to run in
parallel ideally.
Locally, this reduces Liftoff compilation time by more than 20 percent.

R=mstarzinger@chromium.org, mlippautz@chromium.org

Bug: chromium:924956
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Change-Id: I8c522faf385149bfe2cf00d777a7942c537f9cd2
Reviewed-on: https://chromium-review.googlesource.com/c/1477731
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59729}