DevTools: Add parent id to target descriptor in remote debugging.

content/public/browser/devtools_target.h

Index: content/public/browser/devtools_target.h
diff --git a/content/public/browser/devtools_target.h b/content/public/browser/devtools_target.h
index ae4ab8b97e7101f6a6d14a19c9314f31ef5af208..80085350ae1ecebd6e109d269850de57256e9c36 100644
--- a/content/public/browser/devtools_target.h
+++ b/content/public/browser/devtools_target.h
@@ -28,6 +28,9 @@ class DevToolsTarget {
   // Returns the unique target id.
   virtual std::string GetId() const = 0;
 
+  // Returns the id of the parent target, or empty string if no parent.
+  virtual std::string GetParentId() const = 0;
+
   // Returns the target type.
   virtual std::string GetType() const = 0;