sheriff: fix flaky SearchTagRegistryTest.AddAndRemove

This test uses an uninitialized member variable to keep track of
calls.

Bug: None
Change-Id: Ie99dc04857d5b0bf889dcd88a0478d8fef59426b
Tbr: khorimoto@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264584Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782046}

chrome/browser/ui/webui/settings/chromeos/search/search_tag_registry_unittest.cc

@@ -27,7 +27,7 @@ class FakeObserver : public SearchTagRegistry::Observer {
   // SearchTagRegistry::Observer:
   void OnRegistryUpdated() override { ++num_calls_; }
 
-  size_t num_calls_;
+  size_t num_calls_ = 0;
 };
 
 // Note: Copied from printing_section.cc but does not need to stay in sync with