Enable text/plain content tests for browser.

CL:1631149 disabled two integration test for checking if the
FilesApp can make the browser open a file that has been content
sniffed as text/plain (e.g. if the file has no filename extension).

This CL re-renables those tests as this is working correctly
for browser display.

Bug: 772600
Tests: Enabled integration tests again.
Change-Id: Ia6ec09e5b9c3fcf541a95c3906afa8f3ee4533a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645133Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666550}

chrome/browser/chromeos/file_manager/file_manager_browsertest.cc

@@ -341,7 +341,9 @@ WRAPPED_INSTANTIATE_TEST_SUITE_P(
     OpenSniffedFiles, /* open_sniffed_files.js */
     FilesAppBrowserTest,
     ::testing::Values(TestCase("pdfOpenDownloads"),
-                      TestCase("pdfOpenDrive").EnableDriveFs()));
+                      TestCase("pdfOpenDrive").EnableDriveFs(),
+                      TestCase("textOpenDownloads"),
+                      TestCase("textOpenDrive").EnableDriveFs()));
 
 // NaCl fails to compile zip plugin.pexe too often on ASAN, crbug.com/867738
 // The tests are flaky on the debug bot and always time out first and then pass