shell_dialogs: fix SelectFileDialogMacTest on Mojave

On 10.14, a file selection dialog whose default path doesn't exist will default
back to ~/Documents, which breaks this test. To avoid this issue, have this
test use /private/var, which is always present.

Bug: 892174
Change-Id: I3f50a74566bdc5a3bf4eb14433138e65d1cfe2d5
Reviewed-on: https://chromium-review.googlesource.com/c/1316109Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606493}

ui/shell_dialogs/select_file_dialog_mac_unittest.mm

@@ -6,6 +6,7 @@
 
 #include <vector>
 
+#include "base/files/file_util.h"
 #import "base/mac/foundation_util.h"
 #include "base/mac/mac_util.h"
 #include "base/macros.h"
@@ -442,9 +443,8 @@ TEST_F(SelectFileDialogMacTest, MultipleDialogs) {
 
 // Verify that the default_path argument is respected.
 TEST_F(SelectFileDialogMacTest, DefaultPath) {
-  const std::string fake_path = "/fake_directory/filename.txt";
   FileDialogArguments args(GetDefaultArguments());
-  args.default_path = base::FilePath(FILE_PATH_LITERAL(fake_path));
+  args.default_path = base::GetHomeDir().AppendASCII("test.txt");
 
   SelectFileWithParams(args);
   NSSavePanel* panel = GetPanel();