Adds a stress-background-compile mode which runs compilation on a background
thread at the same time as compiling on the main thread to flush out races.
This is added as one of the variants, removing stress_asm_validation since this
is no longer a useful variant.

BUG=v8:5203

Change-Id: I73dd9f21d9b6da4a4cb39c05061aa24116e6c5ad
Reviewed-on: https://chromium-review.googlesource.com/741746
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49376}