Enable GalleryBrowserTestInGuestMode::OpenSingleImageOnDownloads

Test was disabled in issue 804413 due to the linux-chromeos-release bot
flakiness. Re-enable to determine if the test flakes on the CQ.

Tbr: fukino-san
Bug: 804413
Change-Id: I14b1f3dfcb78123a3deadd227960cdb3db91a874
Reviewed-on: https://chromium-review.googlesource.com/981812Reviewed-by: Sasha Morrissey <sashab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546029}

chrome/browser/chromeos/file_manager/gallery_browsertest.cc

@@ -36,9 +36,8 @@ IN_PROC_BROWSER_TEST_F(GalleryBrowserTest,
   StartTest();
 }
 
-// http://crbug.com/804413.
 IN_PROC_BROWSER_TEST_F(GalleryBrowserTestInGuestMode,
-                       DISABLED_OpenSingleImageOnDownloads) {
+                       OpenSingleImageOnDownloads) {
   set_test_case_name("openSingleImageOnDownloads");
   StartTest();
 }