[DevTools] Delete old toolbox on debug reload.

BUG=none

Review URL: https://codereview.chromium.org/967213003

Cr-Commit-Position: refs/heads/master@{#318722}

chrome/browser/devtools/devtools_window.cc

@@ -857,6 +857,8 @@ void DevToolsWindow::WebContentsCreated(WebContents* source_contents,
   if (target_url.SchemeIs(content::kChromeDevToolsScheme) &&
       target_url.path().rfind("toolbox.html") != std::string::npos) {
     CHECK(can_dock_);
+    if (toolbox_web_contents_)
+      delete toolbox_web_contents_;
     toolbox_web_contents_ = new_contents;
   }
 }