Fix NotBlockedExternalUserProviders flakiness on Linux.

Turns out if was a race condition that logged

[FATAL:data_decoder.cc(157)] data_decoder::ServiceProvider::Set()
    must be called before any instances of DataDecoder can be used.

during flakes. Waiting after starting the ExtensionUpdater seems to
fix the issue.

TBR=rdevlin.cronin@chromium.org

Bug: 1023391
Change-Id: Ifc3be2014b7b729ca5a3e091eac1fb2195129bf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1910285
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714579}

chrome/browser/extensions/external_provider_impl_unittest.cc

@@ -139,6 +139,7 @@ class ExternalProviderImplTest : public ExtensionServiceTestBase {
               int(sizeof(prefs)));
     InitializeExtensionService(params);
     service_->updater()->Start();
+    content::RunAllTasksUntilIdle();
   }
 
   // ExtensionServiceTestBase overrides: