Revert "[wasm] Enable guards on 64-bit systems."

Web platform tests that allocate 600 memories fail due to this. (This seems to also interfere with tests on the same shard).

This reverts commit b9abc7f0.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [wasm] Enable guards on 64-bit systems.
> 
> R=​eholk@chromium.org
> 
> Bug: 
> Change-Id: Ia099cf085510da50a87b9f999020d6a66d6eab20
> Reviewed-on: https://chromium-review.googlesource.com/838250
> Commit-Queue: Brad Nelson <bradnelson@chromium.org>
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Eric Holk <eholk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50300}

TBR=bradnelson@chromium.org,eholk@chromium.org,clemensh@chromium.org

Change-Id: Ie7fc2a2142bddb03df41a5b705087a2e1f03e163
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/842925Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50304}

src/wasm/module-compiler.cc

@@ -3174,14 +3174,7 @@ Handle<JSArrayBuffer> InstanceBuilder::AllocateMemory(uint32_t num_pages) {
     thrower_->RangeError("Out of memory: wasm memory too large");
     return Handle<JSArrayBuffer>::null();
   }
-#if V8_TARGET_ARCH_64_BIT && !defined(THREAD_SANITIZER) && \
-    !defined(LEAK_SANITIZER) && !defined(V8_USE_ADDRESS_SANITIZER)
-  // TODO(eholk): Enable sanitizers once we have back-pressure.
-  // Always turn on guard regions in 64-bit.
-  const bool enable_guard_regions = true;
-#else
   const bool enable_guard_regions = use_trap_handler();
-#endif
   const bool is_shared_memory =
       module_->has_shared_memory && i::FLAG_experimental_wasm_threads;
   Handle<JSArrayBuffer> mem_buffer = NewArrayBuffer(

src/wasm/wasm-js.cc

@@ -600,15 +600,8 @@ void WebAssemblyMemory(const v8::FunctionCallbackInfo<v8::Value>& args) {
 
   size_t size = static_cast<size_t>(i::wasm::WasmModule::kPageSize) *
                 static_cast<size_t>(initial);
-#if V8_TARGET_ARCH_64_BIT && !defined(THREAD_SANITIZER) && \
-    !defined(LEAK_SANITIZER) && !defined(V8_USE_ADDRESS_SANITIZER)
-  // TODO(eholk): Enable sanitizers once we have back-pressure.
-  // Always turn on guard regions in 64-bit.
-  const bool enable_guard_regions = true;
-#else
   const bool enable_guard_regions =
       internal::trap_handler::IsTrapHandlerEnabled();
-#endif
   i::Handle<i::JSArrayBuffer> buffer = i::wasm::NewArrayBuffer(
       i_isolate, size, enable_guard_regions,
       is_shared_memory ? i::SharedFlag::kShared : i::SharedFlag::kNotShared);

src/wasm/wasm-objects.cc

@@ -369,17 +369,8 @@ Handle<JSArrayBuffer> GrowMemoryBuffer(Isolate* isolate,
   if (old_pages > maximum_pages || pages > maximum_pages - old_pages) {
     return Handle<JSArrayBuffer>::null();
   }
-#if V8_TARGET_ARCH_64_BIT && !defined(THREAD_SANITIZER) && \
-    !defined(LEAK_SANITIZER) && !defined(V8_USE_ADDRESS_SANITIZER)
-  // TODO(eholk): Enable sanitizers once we have back-pressure.
-  // Always turn on guard regions in 64-bit.
-  // But allow for previously unguarded memory for some cctests.
-  const bool enable_guard_regions =
-      old_buffer.is_null() ? true : old_buffer->has_guard_region();
-#else
   const bool enable_guard_regions =
       old_buffer.is_null() ? use_trap_handler : old_buffer->has_guard_region();
-#endif
   size_t new_size =
       static_cast<size_t>(old_pages + pages) * WasmModule::kPageSize;
   if (new_size > FLAG_wasm_max_mem_pages * WasmModule::kPageSize ||