Skip restoring debug state unless thread previously in DebugScope.

A simpler version of the changes I proposed upstream in this V8 change
request: https://chromium-review.googlesource.com/c/v8/v8/+/1168449

In this version, Debug::RestoreDebug never attempts to enter a new
DebugScope, but merely reuses the previous one, if we're returning to a
thread that was previously in a DebugScope. If the thread was not
previously in a DebugScope, I believe it does not need to have any
debugging state restored with ClearOneShot and PrepareStep.

The tests from https://chromium-review.googlesource.com/c/v8/v8/+/833260
still pass, and the failing V8-CI tests are now passing locally for me:
https://ci.nodejs.org/job/node-test-commit-v8-linux/1601/

Author: Ben Newman

deps/v8/src/debug/debug.cc

@@ -364,16 +364,21 @@ char* Debug::RestoreDebug(char* storage) {
   MemCopy(reinterpret_cast<char*>(&thread_local_), storage,
           ArchiveSpacePerThread());
 
-  // Enter the debugger.
-  DebugScope debug_scope(this);
-
-  // Clear any one-shot breakpoints that may have been set by the other
-  // thread, and reapply breakpoints for this thread.
-  ClearOneShot();
+  if (in_debug_scope()) {
+    // If this thread was in a DebugScope when we archived it, restore the
+    // previous debugging state now. Note that in_debug_scope() returns
+    // true when thread_local_.current_debug_scope_ (restored by MemCopy
+    // above) is non-null.
+
+    // Clear any one-shot breakpoints that may have been set by the other
+    // thread, and reapply breakpoints for this thread.
+    HandleScope scope(isolate_);
+    ClearOneShot();
 
-  if (thread_local_.last_step_action_ != StepNone) {
-    // Reset the previous step action for this thread.
-    PrepareStep(thread_local_.last_step_action_);
+    if (thread_local_.last_step_action_ != StepNone) {
+      // Reset the previous step action for this thread.
+      PrepareStep(thread_local_.last_step_action_);
+    }
   }
 
   return storage + ArchiveSpacePerThread();