[filesapp] Add using DevToolsAgentMap to DevToolsListenerBrowserTest

Add a using directive: a C++ shorthand to describe the contents of the
of agent hosts to devtools listener map.

Tbr: benreich
Bug: 1113941
Change-Id: Iea4b34b056654315d8e0e1382f584326b50122a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484027Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818349}

chrome/browser/chromeos/file_manager/devtools_listener_browsertest.cc

@@ -79,9 +79,12 @@ class DevToolsListenerBrowserTest : public content::DevToolsAgentHostObserver,
 
  private:
   base::ScopedTempDir tmp_dir_;
+
+  using DevToolsAgentMap =  // agent hosts: have a unique devtools listener
+      std::map<content::DevToolsAgentHost*, std::unique_ptr<DevToolsListener>>;
+
+  DevToolsAgentMap devtools_agent_;
   uint32_t process_id_ = 0;
-  std::map<content::DevToolsAgentHost*, std::unique_ptr<DevToolsListener>>
-      devtools_agent_;
 
   DISALLOW_COPY_AND_ASSIGN(DevToolsListenerBrowserTest);
 };