capture_mode: Enable testing file ops in unittests

The test delegate should now return a valid temp
directory for the user's downloads dir.

BUG=none
TEST=Existing tests

Change-Id: I159690b0d3bc330e473c33b52f5057cb823a7363
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587550Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836267}

ash/capture_mode/test_capture_mode_delegate.cc

@@ -6,6 +6,8 @@
 
 #include "ash/services/recording/public/mojom/recording_service.mojom.h"
 #include "base/files/file_path.h"
+#include "base/files/file_util.h"
+#include "base/threading/thread_restrictions.h"
 
 namespace ash {
 
@@ -65,13 +67,17 @@ class FakeRecordingService : public recording::mojom::RecordingService {
 // -----------------------------------------------------------------------------
 // TestCaptureModeDelegate:
 
-TestCaptureModeDelegate::TestCaptureModeDelegate() = default;
+TestCaptureModeDelegate::TestCaptureModeDelegate() {
+  base::ScopedAllowBlockingForTesting allow_blocking;
+  const bool result =
+      base::CreateNewTempDirectory(/*prefix=*/"", &fake_downloads_dir_);
+  DCHECK(result);
+}
 
 TestCaptureModeDelegate::~TestCaptureModeDelegate() = default;
 
 base::FilePath TestCaptureModeDelegate::GetActiveUserDownloadsDir() const {
-  // TODO(afakhry): Add proper code to enable testing.
-  return base::FilePath();
+  return fake_downloads_dir_;
 }
 
 void TestCaptureModeDelegate::ShowScreenCaptureItemInFolder(

ash/capture_mode/test_capture_mode_delegate.h

@@ -9,6 +9,7 @@
 
 #include "ash/public/cpp/capture_mode_delegate.h"
 #include "base/callback.h"
+#include "base/files/file_path.h"
 
 namespace ash {
 
@@ -43,6 +44,7 @@ class TestCaptureModeDelegate : public CaptureModeDelegate {
 
  private:
   std::unique_ptr<FakeRecordingService> fake_service_;
+  base::FilePath fake_downloads_dir_;
 };
 
 }  // namespace ash