[filesapp] Add missing includes and fail if host crashed

Adds #include's that were missing from the initial patch, and FAIL the
test if the DevToolsAgentHost attached to crashes during the test.

Fix HostString() helper to *actually* take an optional prefix: use C++
c++17 {} style initializer.

Test: browser_test --gtest_filter="*DevToolsListener*"
Bug: 1113941
Change-Id: I5901019add9fea92b6e3a8b5b881b8d3d197b6c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2465456
Commit-Queue: Ben Reich <benreich@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816070}

chrome/browser/chromeos/file_manager/devtools_listener.cc

@@ -72,7 +72,7 @@ void DevToolsListener::Detach(content::DevToolsAgentHost* host) {
 }
 
 std::string DevToolsListener::HostString(content::DevToolsAgentHost* host,
-                                         const std::string& prefix = "") {
+                                         const std::string& prefix) {
   std::string result = base::StrCat(
       {prefix, " ", host->GetType(), " title: ", host->GetTitle()});
   std::string description = host->GetDescription();

chrome/browser/chromeos/file_manager/devtools_listener.h

@@ -42,7 +42,7 @@ class DevToolsListener : public content::DevToolsAgentHostClient {
   // Returns a string that uniquely identifies a host
   // with an optional prefix.
   static std::string HostString(content::DevToolsAgentHost* host,
-                                const std::string& prefix);
+                                const std::string& prefix = {});
 
  private:
   // Enable CDP on host.

chrome/browser/chromeos/file_manager/devtools_listener_browsertest.cc

@@ -3,16 +3,20 @@
 // found in the LICENSE file.
 
 #include <map>
+#include <memory>
 
 #include "base/files/file_util.h"
+#include "base/files/scoped_temp_dir.h"
 #include "base/path_service.h"
 #include "base/process/process_handle.h"
+#include "base/threading/thread_restrictions.h"
 #include "chrome/browser/chromeos/file_manager/devtools_listener.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/chrome_paths.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/ui_test_utils.h"
+#include "content/public/browser/devtools_agent_host.h"
 #include "content/public/browser/devtools_agent_host_observer.h"
 #include "content/public/test/browser_test.h"
 #include "content/public/test/browser_test_utils.h"
@@ -55,7 +59,7 @@ class DevToolsListenerBrowserTest : public content::DevToolsAgentHostObserver,
                                 base::TerminationStatus status) override {
     if (devtools_agent_.find(host) == devtools_agent_.end())
       return;
-    NOTREACHED();
+    LOG(FATAL) << "Host crashed: " << DevToolsListener::HostString(host);
   }
 
   void CollectCodeCoverage() {