Reland "Default to the threadsafe GTest death_test_style."

This is a reland of b579146695ba0c045d26b022e63d199bd23f8349

Original change's description:
> Default to the threadsafe GTest death_test_style.
>
> Using the default style of "fast" can lead to subtle test flakiness.
> And on macOS, most of CoreFoundation is not safe to use after fork()
> without exec(). Several //base implementations on Mac use CF, and so it
> is almost never safe to use the "fast" death_test_style.
>
> Change-Id: Idcee977978561e0a57ace20b53f597c566708692
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/570500
> Commit-Queue: Robert Sesek <rsesek@chromium.org>
> Reviewed-by: danakj <danakj@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#854588}

Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2747682
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#862111}
Message-Id: Merged from chromium b374bd6f372ac3ead20ed8b3a53ad1b60bd26b50
Change-Id: I9abd7bae0c74f2537405b21dbaa4c6582cbab6df
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2818148
Reviewed-by: Mark Mentovai <mark@chromium.org>
GitOrigin-RevId: ae20ed4321210ae6ef5ae810fb94610205a84ec0
diff --git a/test/multiprocess_posix_test.cc b/test/multiprocess_posix_test.cc
index 9489147..b866695 100644
--- a/test/multiprocess_posix_test.cc
+++ b/test/multiprocess_posix_test.cc
@@ -155,6 +155,8 @@
       : Multiprocess(),
         who_closes_(who_closes),
         what_closes_(what_closes) {
+    // Fails under "threadsafe" mode on macOS 10.11.
+    testing::GTEST_FLAG(death_test_style) = "fast";
   }
 
   ~TestMultiprocessClosePipe() {}