Revert "Disable ExternalProviderImplTest.BlockedExternalUserProviders on Linux."

This reverts commit 5e7d8eb8.

Reason for revert: This CL disabled the wrong test.

Original change's description:
> Disable ExternalProviderImplTest.BlockedExternalUserProviders on Linux.
> 
> Disabling this test to reduce jank on the bots.
> 
> TBR=domfc@chromium.org
> 
> Bug: 1023391
> Change-Id: I26243bb6acba2af947357222ac1fa5664601b1e0
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1908063
> Reviewed-by: James MacLean <wjmaclean@chromium.org>
> Commit-Queue: James MacLean <wjmaclean@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#714232}

TBR=wjmaclean@chromium.org,domfc@chromium.org

Change-Id: Ice16cb05e8e047380bba533e9b0cfda37204f316
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1023391
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1909642Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714272}

chrome/browser/extensions/external_provider_impl_unittest.cc

@@ -230,13 +230,7 @@ TEST_F(ExternalProviderImplTest, InAppPayments) {
       extension_misc::kInAppPaymentsSupportAppId));
 }
 
-// https://crbug.com/1023391
-#if defined(OS_LINUX)
-#define MAYBE_BlockedExternalUserProviders DISABLED_BlockedExternalUserProviders
-#else
-#define MAYBE_BlockedExternalUserProviders BlockedExternalUserProviders
-#endif
-TEST_F(ExternalProviderImplTest, MAYBE_BlockedExternalUserProviders) {
+TEST_F(ExternalProviderImplTest, BlockedExternalUserProviders) {
   OverrideExternalExtensionsPath();
   InitServiceWithExternalProviders(true);