Remove useless MAYBE definition from NestedURLDownloadsToExtensionAllowed

The test appears to already be enabled everywhere without generating
complaints about flakiness, and the MAYBE definition isn't used.

Bug: 806684
Change-Id: I02b61b79cfce047be4c021401f41298f9d1aef3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642673
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Auto-Submit: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666055}

chrome/browser/extensions/process_manager_browsertest.cc

@@ -915,14 +915,6 @@ IN_PROC_BROWSER_TEST_F(ProcessManagerBrowserTest,
   }
 }
 
-// Flaky on Win, Mac and Linux (http://crbug.com/806684).
-#if defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_LINUX)
-#define MAYBE_NestedURLDownloadsToExtensionAllowed \
-  DISABLED_NestedURLDownloadsToExtensionAllowed
-#else
-#define MAYBE_NestedURLDownloadsToExtensionAllowed \
-  NestedURLDownloadsToExtensionAllowed
-#endif
 // Check that browser-side restrictions on extension blob/filesystem URLs allow
 // navigations that will result in downloads.  See https://crbug.com/714373.
 IN_PROC_BROWSER_TEST_F(ProcessManagerBrowserTest,