Enable text/plain content test for quickview.

CL:1631149 disabled the integration test for checking if the
quickview panel in FilesApp can display content in a file
that has been content sniffed as text/plain (e.g. if the
file has no filename extension).

This CL re-renables that test as this is working correctly
for quickview.

Bug: 772600
Tests: Enabled integration test again.
Change-Id: I69bf2feefe5e28860dfd62a47593cb179256bf46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1644856Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666539}

chrome/browser/chromeos/file_manager/file_manager_browsertest.cc

@@ -520,6 +520,7 @@ WRAPPED_INSTANTIATE_TEST_SUITE_P(
 #endif
         TestCase("openQuickViewKeyboardUpDownChangesView"),
         TestCase("openQuickViewKeyboardLeftRightChangesView"),
+        TestCase("openQuickViewSniffedText"),
         TestCase("openQuickViewScrollText"),
         TestCase("openQuickViewScrollHtml"),
         TestCase("openQuickViewBackgroundColorText"),

chrome/test/data/chromeos/file_manager/plaintext

+From somewhere a long time ago.

ui/file_manager/integration_tests/test_util.js

@@ -673,6 +673,17 @@ var ENTRIES = {
     typeText: 'Plain text',
   }),
 
+  // Note, no MIME type here, this is used for testing file content sniffing.
+  plainText: new TestEntryInfo({
+    type: EntryType.FILE,
+    sourceFileName: 'plaintext',
+    targetPath: 'plaintext',
+    lastModifiedTime: 'Sep 4, 1998, 12:34 PM',
+    nameText: 'plaintext',
+    sizeText: '32 bytes',
+    typeText: 'Plain text',
+  }),
+
   tallHtml: new TestEntryInfo({
     type: EntryType.FILE,
     sourceFileName: 'tall.html',