Expose the full API for attaching devtools clients:

headless/public/headless_devtools_target.h

Index: headless/public/headless_devtools_target.h
diff --git a/headless/public/headless_devtools_target.h b/headless/public/headless_devtools_target.h
index 941fe6f51e2ae640e9462d015f5c646005eebc5e..e8047f39b8f131432fed075bf8ac0222ac9c613c 100644
--- a/headless/public/headless_devtools_target.h
+++ b/headless/public/headless_devtools_target.h
@@ -20,14 +20,21 @@ class HEADLESS_EXPORT HeadlessDevToolsTarget {
   // Attach or detach a client to this target. A client must be attached in
   // order to send commands or receive notifications from the target.
   //
-  // A single client may be attached to at most one target at a time. Note that
-  // currently also only one client may be attached to a single target at a
-  // time.
+  // A single client may be attached to at most one target at a time. If
+  // the target already has a client attached, AttachClient will return false.
+  // ForceAttachClient will detach any existing connection before attaching
+  // |client|.
+  // Note that currently also only one client may be attached to a single target
+  // at a time.
   //
   // |client| must outlive this target.
-  virtual void AttachClient(HeadlessDevToolsClient* client) = 0;
+  virtual bool AttachClient(HeadlessDevToolsClient* client) = 0;
+  virtual void ForceAttachClient(HeadlessDevToolsClient* client) = 0;
   virtual void DetachClient(HeadlessDevToolsClient* client) = 0;
 
+  // Returns true if a devtools client is attached.
+  virtual bool IsAttached() = 0;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(HeadlessDevToolsTarget);
 };