[wasm] Avoid code modifications during code freeing

Code freeing can happen at any point in time where a GC might be
triggered. Hence it's difficult to ensure that no other
{CodeSpaceWriteScope} is already open at that point. The way these scope
objects are implemented forbids multiple scopes for different modules
though.
To solve this, this CL just avoids the code zapping in
{WasmCodeAllocator::FreeCode}, which is the only place that actually
writes to the code space. Without this, we do not need the
{CodeSpaceWriteScope} in {NativeModule::FreeCode} any more.

R=jkummerow@chromium.org

Bug: v8:11974
Change-Id: I1f01979e1eaea6c311c9ad568d605aabeef3bfc4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3081522
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76174}

src/wasm/wasm-code-manager.cc

@@ -822,13 +822,6 @@ void WasmCodeAllocator::FreeCode(base::Vector<WasmCode* const> codes) {
   DisjointAllocationPool freed_regions;
   size_t code_size = 0;
   for (WasmCode* code : codes) {
-    // TODO(clemensb): If zapping is worth it, we need to unprotect the code
-    // memory first.
-    if (!protect_code_memory_) {
-      ZapCode(code->instruction_start(), code->instructions().size());
-      FlushInstructionCache(code->instruction_start(),
-                            code->instructions().size());
-    }
     code_size += code->instructions().size();
     freed_regions.Merge(base::AddressRegion{code->instruction_start(),
                                             code->instructions().size()});
@@ -2356,10 +2349,6 @@ std::vector<int> NativeModule::FindFunctionsToRecompile(
 
 void NativeModule::FreeCode(base::Vector<WasmCode* const> codes) {
   base::RecursiveMutexGuard guard(&allocation_mutex_);
-  // Get writable permission already here (and not inside the loop in
-  // {WasmCodeAllocator::FreeCode}), to avoid switching for each {code}
-  // individually.
-  CodeSpaceWriteScope code_space_write_scope(this);
   // Free the code space.
   code_allocator_.FreeCode(codes);