[unwinder] Copy constructor cleanups

There were some cleanups to be done after crrev.com/c/v8/v8/+/2472000/
was merged.

Bug: v8:10799
Change-Id: I09bc2d123f89b88c74c3aecfa97c82d1925a1f2b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2488686Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70685}

src/api/api.cc

@@ -11154,14 +11154,7 @@ RegisterState::RegisterState()
 RegisterState::~RegisterState() = default;
 
 RegisterState::RegisterState(const RegisterState& other) V8_NOEXCEPT {
-  pc = other.pc;
-  sp = other.sp;
-  fp = other.fp;
-  lr = other.lr;
-  if (other.callee_saved) {
-    callee_saved =
-        std::make_unique<CalleeSavedRegisters>(*(other.callee_saved));
-  }
+  *this = other;
 }
 
 RegisterState& RegisterState::operator=(const RegisterState& other)
@@ -11172,8 +11165,12 @@ RegisterState& RegisterState::operator=(const RegisterState& other)
     fp = other.fp;
     lr = other.lr;
     if (other.callee_saved) {
+      // Make a deep copy if {other.callee_saved} is non-null.
       callee_saved =
           std::make_unique<CalleeSavedRegisters>(*(other.callee_saved));
+    } else {
+      // Otherwise, set {callee_saved} to null to match {other}.
+      callee_saved.reset();
     }
   }
   return *this;