Speculative fix for crash on MTP directory read.

Within MTPDeviceDelegateImplLinux::ReadDirectoryInternal, the call to
PendingRequestDone at the end of the function allowed for another task
to run and potentially delete the file_id from the cache prior to the
directory listing being done. This would then cause a nullptr
dereference in OnDidReadDirectory which expects the cache entry to
exist. Since ReadDirectoryInternal is only run from a task, just
continue the task instead of posting it to the request queue.

BUG=817523

Change-Id: I9df651864ba1ef3c26365832b149871b027f99d5
Reviewed-on: https://chromium-review.googlesource.com/1195202Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Anand Mistry <amistry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587857}

chrome/browser/media_galleries/chromeos/mtp_device_delegate_impl_chromeos.cc

@@ -955,9 +955,15 @@ void MTPDeviceDelegateImplLinux::ReadDirectoryInternal(
     const ReadDirectorySuccessCallback& success_callback,
     const ErrorCallback& error_callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
+  DCHECK(task_in_progress_);
 
   base::Optional<uint32_t> dir_id = CachedPathToId(root);
-  if (dir_id) {
+  if (!dir_id) {
+    error_callback.Run(base::File::FILE_ERROR_NOT_FOUND);
+    PendingRequestDone();
+    return;
+  }
+
   GetFileInfoSuccessCallback success_callback_wrapper =
       base::Bind(&MTPDeviceDelegateImplLinux::OnDidGetFileInfoToReadDirectory,
                  weak_ptr_factory_.GetWeakPtr(), *dir_id, success_callback,
@@ -968,14 +974,9 @@ void MTPDeviceDelegateImplLinux::ReadDirectoryInternal(
   base::Closure closure =
       base::Bind(&GetFileInfoOnUIThread, storage_name_, read_only_, *dir_id,
                  success_callback_wrapper, error_callback_wrapper);
-    EnsureInitAndRunTask(PendingTaskInfo(base::FilePath(),
-                                         content::BrowserThread::UI,
-                                         FROM_HERE,
-                                         closure));
-  } else {
-    error_callback.Run(base::File::FILE_ERROR_NOT_FOUND);
-  }
-  PendingRequestDone();
+
+  content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
+                                   closure);
 }
 
 void MTPDeviceDelegateImplLinux::CreateSnapshotFileInternal(