Expose the full API for attaching devtools clients:

headless/lib/browser/headless_web_contents_impl.cc

Index: headless/lib/browser/headless_web_contents_impl.cc
diff --git a/headless/lib/browser/headless_web_contents_impl.cc b/headless/lib/browser/headless_web_contents_impl.cc
index dabdee17482ebde14bec6bffdc7e108068ecac94..014e48240c9421b45e62f93a59174163738875a6 100644
--- a/headless/lib/browser/headless_web_contents_impl.cc
+++ b/headless/lib/browser/headless_web_contents_impl.cc
@@ -240,8 +240,15 @@ HeadlessDevToolsTarget* HeadlessWebContentsImpl::GetDevToolsTarget() {
   return web_contents()->GetMainFrame()->IsRenderFrameLive() ? this : nullptr;
 }
 
-void HeadlessWebContentsImpl::AttachClient(HeadlessDevToolsClient* client) {
-  HeadlessDevToolsClientImpl::From(client)->AttachToHost(agent_host_.get());
+bool HeadlessWebContentsImpl::AttachClient(HeadlessDevToolsClient* client) {
+  return HeadlessDevToolsClientImpl::From(client)->AttachToHost(
+      agent_host_.get());
+}
+
+void HeadlessWebContentsImpl::ForceAttachClient(
+    HeadlessDevToolsClient* client) {
+  HeadlessDevToolsClientImpl::From(client)->ForceAttachToHost(
+      agent_host_.get());
 }
 
 void HeadlessWebContentsImpl::DetachClient(HeadlessDevToolsClient* client) {
@@ -249,6 +256,11 @@ void HeadlessWebContentsImpl::DetachClient(HeadlessDevToolsClient* client) {
   HeadlessDevToolsClientImpl::From(client)->DetachFromHost(agent_host_.get());
 }
 
+bool HeadlessWebContentsImpl::IsAttached() {
+  DCHECK(agent_host_);
+  return agent_host_->IsAttached();
+}
+
 content::WebContents* HeadlessWebContentsImpl::web_contents() const {
   return web_contents_.get();
 }