Revert "Oilpan: Replace Main/WorkerThreadMutex with one ActiveThreadMutex"

This reverts commit dc3fde2e427aa046fd4f0a7a16f8fd72b9c6a96b.

Reason for revert: fast/js/try-catch-crash.html is burning on WebKit Linux Trusty MSAN; this is a suspect

Original change's description:
> Oilpan: Replace Main/WorkerThreadMutex with one ActiveThreadMutex
> 
> TSAN reported lock-order-inversion because we were using two mutexes. Simplify the code to use just one.
> 
> Bug: 915200
> Change-Id: I5dbda4ca21cd4f5fa95ecb1c9b51922262798169
> Reviewed-on: https://chromium-review.googlesource.com/c/1384384
> Commit-Queue: Keishi Hattori <keishi@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#618754}

TBR=haraken@chromium.org,keishi@chromium.org,mlippautz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 915200
Change-Id: If4be6a8156292d27790a7f12a62f2d559c730236
Reviewed-on: https://chromium-review.googlesource.com/c/1390426
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618828}
3 files changed