Revert "Remove trap handler fallback for sanitizers"

This reverts commit 26a78061.

Reason for revert: Not all fuzzers support custom segfault handlers yet, see https://crbug.com/918949

Original change's description:
> Remove trap handler fallback for sanitizers
> 
> Since https://crrev.com/c/1335572, our sanitizers allow to set custom
> segfault handlers. Thus remove special code that was added to handle
> sanitizers that prevent installation of segfault handlers. Instead,
> CHECK that the signal handler was installed correctly.
> 
> R=​ahaas@chromium.org, mseaborn@chromium.org, mark@chromium.org
> 
> Bug: chromium:830894
> Change-Id: I3bd66e33efdceb3e8469f3f4a09fbde90cb3d7ec
> Reviewed-on: https://chromium-review.googlesource.com/c/1392199
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#58513}

TBR=mseaborn@chromium.org,ahaas@chromium.org,mark@chromium.org,clemensh@chromium.org

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

Bug: chromium:830894, chromium:918949
Change-Id: Ide545860cf7729139ac50c0dd2e85facca49b0b1
Reviewed-on: https://chromium-review.googlesource.com/c/1396277Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58556}

src/trap-handler/handler-outside-posix.cc

@@ -13,7 +13,7 @@
 //    should be as self-contained as possible to make it easy to audit the code.
 //
 // 2. Any changes must be reviewed by someone from the crash reporting
-//    or security team. See OWNERS for suggested reviewers.
+//    or security team. Se OWNERS for suggested reviewers.
 //
 // For more information, see https://goo.gl/yMeyUY.
 //
@@ -52,21 +52,36 @@ bool RegisterDefaultTrapHandler() {
     return false;
   }
 
-  // Check that the signal handler is indeed registered. Sanitizers for example
-  // might prevent this, and then report crashes later because signals are not
-  // being caught correctly.
+// Sanitizers often prevent us from installing our own signal handler. Attempt
+// to detect this and if so, refuse to enable trap handling.
+//
+// TODO(chromium:830894): Remove this once all bots support custom signal
+// handlers.
+#if defined(ADDRESS_SANITIZER) || defined(MEMORY_SANITIZER) || \
+    defined(THREAD_SANITIZER) || defined(LEAK_SANITIZER) ||    \
+    defined(UNDEFINED_SANITIZER)
   struct sigaction installed_handler;
   CHECK_EQ(sigaction(kOobSignal, NULL, &installed_handler), 0);
-  CHECK_EQ(HandleSignal, installed_handler.sa_sigaction);
+  // If the installed handler does not point to HandleSignal, then
+  // allow_user_segv_handler is 0.
+  if (installed_handler.sa_sigaction != HandleSignal) {
+    printf(
+        "WARNING: sanitizers are preventing signal handler installation. "
+        "Trap handlers are disabled.\n");
+    return false;
+  }
+#endif
 
   g_is_default_signal_handler_registered = true;
   return true;
 }
 
 void RemoveTrapHandler() {
-  if (!g_is_default_signal_handler_registered) return;
-  if (sigaction(kOobSignal, &g_old_handler, nullptr) != 0) return;
-  g_is_default_signal_handler_registered = false;
+  if (g_is_default_signal_handler_registered) {
+    if (sigaction(kOobSignal, &g_old_handler, nullptr) == 0) {
+      g_is_default_signal_handler_registered = false;
+    }
+  }
 }
 #endif  // V8_TRAP_HANDLER_SUPPORTED