Fix memory leak in ExternalProviderImplChromeOSTest.

My recent CL
https://chromium-review.googlesource.com/c/chromium/src/+/870744
caused a leak in ExternalProviderImplChromeOSTest as the test shuts down
with some extension installation pending.
Make sure we wait for all extensions to be installed before we exit the
test.

Bug: 805591
Change-Id: I89c1187d9e32ce0ab859689286c6283b0316d602
Reviewed-on: https://chromium-review.googlesource.com/884270Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531731}

chrome/browser/extensions/external_provider_impl_chromeos_unittest.cc

@@ -95,6 +95,12 @@ class ExternalProviderImplChromeOSTest : public ExtensionServiceTestBase {
   }
 
   void TearDown() override {
+    // If some extensions are being installed (on a background thread) and we
+    // stop before the intsallation is complete, some installation related
+    // objects might be leaked (as the background thread won't block on exit and
+    // finish cleanly).
+    // So ensure we let pending extension installations finish.
+    WaitForPendingStandaloneExtensionsInstalled();
     chromeos::KioskAppManager::Shutdown();
   }