[wasm] Ignore trap handler flag in IsTrapHandlerEnabled

This CL also deprecates V8::RegisterDefaultSignalHandler. Now instead of
using the old API, clients should call V8::EnableWebAssemblyTrapHandler.
Just setting the --wasm-trap-handler flag will no longer have any
effect.

Bug: v8:5277
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Idd862185af9abcd4a3c845c02f9e916e8b56f114
Reviewed-on: https://chromium-review.googlesource.com/965005Reviewed-by: Karl Schimpf <kschimpf@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52126}

src/trap-handler/trap-handler.h

@@ -75,8 +75,7 @@ bool EnableTrapHandler(bool use_v8_signal_handler);
 
 inline bool IsTrapHandlerEnabled() {
   DCHECK_IMPLIES(g_is_trap_handler_enabled, V8_TRAP_HANDLER_SUPPORTED);
-  return (V8_TRAP_HANDLER_SUPPORTED && FLAG_wasm_trap_handler) ||
-         g_is_trap_handler_enabled;
+  return g_is_trap_handler_enabled;
 }
 
 extern THREAD_LOCAL int g_thread_in_wasm_code;