[headless] Call StopLocalDevToolsHttpHandler only when needed.

headless/lib/browser/headless_browser_main_parts.h

Index: headless/lib/browser/headless_browser_main_parts.h
diff --git a/headless/lib/browser/headless_browser_main_parts.h b/headless/lib/browser/headless_browser_main_parts.h
index 43d396839dd110467701baacca6504e21a8f6e79..97bc9e9e9120be4797fe856212fa4525a8c4a410 100644
--- a/headless/lib/browser/headless_browser_main_parts.h
+++ b/headless/lib/browser/headless_browser_main_parts.h
@@ -27,6 +27,8 @@ class HeadlessBrowserMainParts : public content::BrowserMainParts {
  private:
   HeadlessBrowserImpl* browser_;  // Not owned.
 
+  bool devtools_http_handler_started_;
+
   DISALLOW_COPY_AND_ASSIGN(HeadlessBrowserMainParts);
 };