[Mac] Specify the MAP_JIT flag when allocating pages for V8.

In the future, macOS executables will be signed with the "runtime"
option, which restricts the execution of writable memory. By
specifying the MAP_JIT flag to mmap and code signing with the
"com.apple.security.cs.allow-jit" entitlement, writable memory can be
executed.

Neither the runtime option or entitlement are currently specified during
signing, but setting the MAP_JIT flag is harmless. The signing options
will be specified in follow-up CLs.

Bug: 850193
Change-Id: I834a75e65b815d5a9a62ef1e3d00e143b36d2ce4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613809
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660935}

base/allocator/partition_allocator/page_allocator_internals_posix.h

@@ -12,6 +12,8 @@
 #include "build/build_config.h"
 
 #if defined(OS_MACOSX)
+#include "base/mac/mac_util.h"
+
 #include <mach/mach.h>
 #endif
 #if defined(OS_LINUX)
@@ -66,12 +68,21 @@ void* SystemAllocPagesInternal(void* hint,
   int access_flag = GetAccessFlags(accessibility);
   int map_flags = MAP_ANONYMOUS | MAP_PRIVATE;
 
-  // TODO(https://crbug.com/927411): Remove once Fuchsia uses a native page
-  // allocator, rather than relying on POSIX compatibility.
+  // TODO(https://crbug.com/927411): Remove OS_FUCHSIA once Fuchsia uses a
+  // native page allocator, rather than relying on POSIX compatibility.
 #if defined(OS_FUCHSIA)
   if (page_tag == PageTag::kV8) {
     map_flags |= MAP_JIT;
   }
+#elif defined(OS_MACOSX)
+  // On macOS 10.14 and higher, executables that are code signed with the
+  // "runtime" option cannot execute writable memory by default. They can opt
+  // into this capability by specifying the "com.apple.security.cs.allow-jit"
+  // code signing entitlement and allocating the region with the MAP_JIT flag.
+  static const bool kNeedMapJIT = mac::IsAtLeastOS10_14();
+  if (page_tag == PageTag::kV8 && kNeedMapJIT) {
+    map_flags |= MAP_JIT;
+  }
 #endif
 
   void* ret =