DevTools: Don't throw an error when filesystems are re-added

This removes and re-adds a filesystem if it was already added instead
of throwing an error. It makes it easier to work with overrides if
you are coming from an old version of overrides or add the folder to
the wrong type of persistence.

Bug: 760316
Change-Id: I8578fef29eba2529ded714cf8bf8582d616c439d
Reviewed-on: https://chromium-review.googlesource.com/820569Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523296}

chrome/browser/devtools/devtools_file_helper.cc

@@ -56,7 +56,6 @@ namespace {
 
 static const char kRootName[] = "<root>";
 static const char kPermissionDenied[] = "<permission denied>";
-static const char kAlreadyAdded[] = "<already added>";
 
 base::LazyInstance<base::FilePath>::Leaky
     g_last_save_path = LAZY_INSTANCE_INITIALIZER;
@@ -343,10 +342,8 @@ void DevToolsFileHelper::InnerAddFileSystem(
     const base::FilePath& path) {
   std::string file_system_path = path.AsUTF8Unsafe();
 
-  if (IsFileSystemAdded(file_system_path)) {
-    FailedToAddFileSystem(kAlreadyAdded);
-    return;
-  }
+  if (IsFileSystemAdded(file_system_path))
+    RemoveFileSystem(file_system_path);
 
   std::string path_display_name = path.AsEndingWithSeparator().AsUTF8Unsafe();
   base::string16 message = l10n_util::GetStringFUTF16(