ppapi: disable FileChooser_Quarantine test on Mac

This test doesn't work on Mac and probably won't ever be made to work.
Issue 899286 obscured this test breakage from us, also.

Bug: 1028239
Change-Id: I9f9865ba7ea4fd626fe50298e60c772876b0439b
Tbr: raymes@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943473Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720591}

chrome/test/ppapi/ppapi_filechooser_browsertest.cc

@@ -236,12 +236,19 @@ IN_PROC_BROWSER_TEST_F(PPAPIFileChooserTest, FileChooser_SaveAs_Cancel) {
   RunTestViaHTTP("FileChooser_SaveAsCancel");
 }
 
-#if defined(OS_WIN) || defined(OS_MACOSX)
-// On Windows and macOS, tests that a file downloaded via PPAPI FileChooser API
-// has the mark-of-the-web. The PPAPI FileChooser implementation invokes
-// QuarantineFile in order to mark the file as being downloaded from the web as
-// soon as the file is created. This MotW prevents the file being opened without
-// due security warnings if the file is executable.
+#if defined(OS_WIN)
+// On Windows, tests that a file downloaded via PPAPI FileChooser API has the
+// mark-of-the-web. The PPAPI FileChooser implementation invokes QuarantineFile
+// in order to mark the file as being downloaded from the web as soon as the
+// file is created. This MotW prevents the file being opened without due
+// security warnings if the file is executable.
+//
+// This test is disabled on Mac even though quarantine support is implemented on
+// Mac. New files created on Mac only receive the MOTW if the process creating
+// them comes from an app with LSFileQuarantineEnabled in its Info.plist. Since
+// PPAPI delegates file creation to the browser process, which in browser_tests
+// is not part of an app, files downloaded by them do not receive the MOTW and
+// this test fails.
 IN_PROC_BROWSER_TEST_F(PPAPIFileChooserTest, FileChooser_Quarantine) {
   base::ScopedAllowBlockingForTesting allow_blocking;
   base::ScopedTempDir temp_dir;