Revert "[sheriff] Disable the flaky test on linux."

This reverts commit bae24cf6.

Reason for revert: https://crbug.com/1141271 refers to content_browsertests, but CheckHistograms runs in unit_tests

I suspect the red herring came from https://crrev.com/c/2489020
which was reverted at a similar time and shows up in the
flakiness dashboard.

Original change's description:
> [sheriff] Disable the flaky test on linux.
>
> AboutFlagsHistogramTest.CheckHistograms is flaky on linux.
>
> TBR=asvitkine@chromium.org,ellyjones@chromium.org
>
> Bug: 1141271
> Change-Id: Ifbda96854d6b9658dd27c5d1f90f92e39347fd4c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491701
> Commit-Queue: Hayato Ito <hayato@chromium.org>
> Reviewed-by: Hayato Ito <hayato@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#819699}

TBR=ellyjones@chromium.org,asvitkine@chromium.org,hayato@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Id01d2b1ca4ef59f4d0d41d7495fec7d2d2413493
Fixed: 1141271
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507229
Auto-Submit: Trent Apted <tapted@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822148}

chrome/browser/about_flags_unittest.cc

@@ -151,14 +151,7 @@ class AboutFlagsHistogramTest : public ::testing::Test {
   }
 };
 
-// http://crbug.com/1141271
-#if defined(OS_LINUX)
-#define MAYBE_CheckHistograms DISABLED_CheckHistograms
-#else
-#define MAYBE_CheckHistograms CheckHistograms
-#endif
-
-TEST_F(AboutFlagsHistogramTest, MAYBE_CheckHistograms) {
+TEST_F(AboutFlagsHistogramTest, CheckHistograms) {
   base::Optional<base::HistogramEnumEntryMap> login_custom_flags =
       base::ReadEnumFromEnumsXml("LoginCustomFlags");
   ASSERT_TRUE(login_custom_flags)