Reinit PnaclHost taskRunner for tests.

The purpose of this cl is to prevent the tests from crashing
on all platforms when running in a batch.

The Task Runner outlives the PooledTaskRunnerDelegate,
which it holds a reference too.
This causes failure in batch test run.

Bug: 1002573
Change-Id: I5fca10116e7b40a2b70bf867d55165b8b5825708
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1794303Reviewed-by: Derek Schuff <dschuff@chromium.org>
Commit-Queue: Ilia Samsonov <isamsonov@google.com>
Cr-Commit-Position: refs/heads/master@{#695256}

components/nacl/browser/pnacl_host.cc

@@ -158,9 +158,13 @@ void PnaclHost::Init() {
 }
 
 // Initialize for testing, optionally using the in-memory backend, and manually
-// setting the temporary file directory instead of using the system directory.
+// setting the temporary file directory instead of using the system directory,
+// and re-initializing file task runner.
 void PnaclHost::InitForTest(base::FilePath temp_dir, bool in_memory) {
   DCHECK(thread_checker_.CalledOnValidThread());
+  file_task_runner_ =
+      base::CreateSequencedTaskRunner({base::ThreadPool(), base::MayBlock(),
+                                       base::TaskPriority::USER_VISIBLE});
   disk_cache_.reset(new PnaclTranslationCache());
   cache_state_ = CacheInitializing;
   temp_dir_ = temp_dir;