[wx] Keep the execute bit while making code writable

Otherwise SMC perturbs the icache too much it seems.

Change-Id: Iceea779a7e3deee90efe29df568732d6ac5bcffa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2753768
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73355}

include/v8-platform.h

@@ -408,7 +408,6 @@ class PageAllocator {
     kNoAccess,
     kRead,
     kReadWrite,
-    // TODO(hpayer): Remove this flag. Memory should never be rwx.
     kReadWriteExecute,
     kReadExecute,
     // Set this when reserving memory that will later require kReadWriteExecute

src/heap/memory-chunk.cc

@@ -91,7 +91,7 @@ void MemoryChunk::SetReadAndWritable() {
     DCHECK(IsAligned(unprotect_start, page_size));
     size_t unprotect_size = RoundUp(area_size(), page_size);
     CHECK(reservation_.SetPermissions(unprotect_start, unprotect_size,
-                                      PageAllocator::kReadWrite));
+                                      PageAllocator::kReadWriteExecute));
   }
 }