[compiler] Explicitly initialize const member

This fixes a compilation error with Xcode.

Change-Id: Iaab7cab0d3200c5622f306c7ad0926931d5dce47
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1948716Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Cr-Commit-Position: refs/heads/master@{#65327}

src/compiler/serializer-for-background-compilation.cc

@@ -309,7 +309,7 @@ class VirtualClosure {
 class CompilationSubject {
  public:
   explicit CompilationSubject(VirtualClosure virtual_closure)
-      : virtual_closure_(virtual_closure) {}
+      : virtual_closure_(virtual_closure), closure_() {}
 
   // The zone parameter is to correctly initialize the virtual closure,
   // which contains zone-allocated context information.