V4LDM: Add check for HashPrefix match and some tests

components/safe_browsing_db/v4_local_database_manager.h

Index: components/safe_browsing_db/v4_local_database_manager.h
diff --git a/components/safe_browsing_db/v4_local_database_manager.h b/components/safe_browsing_db/v4_local_database_manager.h
index 0bc033f0381e6152e830181729f56f87ac8cbbf8..a6506b15f2f975a9e82d27f9016f32afea639058 100644
--- a/components/safe_browsing_db/v4_local_database_manager.h
+++ b/components/safe_browsing_db/v4_local_database_manager.h
@@ -58,6 +58,29 @@ class V4LocalDatabaseManager : public SafeBrowsingDatabaseManager {
   bool IsCsdWhitelistKillSwitchOn() override;
 
  private:
+  // Clients queued up for checking once the database is ready.
+  struct QueuedCheck {

[Nathan Parker, 2016/08/15 19:06:41]

It'd be better to add queuing all at once in a separate CL, since I can't tell
what the full impl will be.

[Scott Hess - ex-Googler, 2016/08/15 20:57:39]

I agree.  The queued checks in the prior version felt bolted-on to me, I always
wondered if there were a better way to handle them.  That kind of thing is
probably best to handle all at once, rather than dealing with it after you've
landed a bunch of scaffolding.

[vakh (use Gerrit instead), 2016/08/17 18:23:06]

Done.

+    QueuedCheck(const ListType check_type,
+                Client* client,
+                const GURL& url,
+                const std::vector<UpdateListIdentifier>& expected_threats,
+                const base::TimeTicks& start);
+    QueuedCheck(const QueuedCheck& other);
+    ~QueuedCheck();
+    ListType check_type;
+    Client* client;
+    GURL url;
+    std::vector<UpdateListIdentifier> expected_threats;
+    base::TimeTicks start;  // When check was queued.
+  };
+
+ private:
+  friend class V4LocalDatabaseManagerTest;
+  void SetTaskRunnerForTest(
+      const scoped_refptr<base::SequencedTaskRunner>& task_runner) {
+    task_runner_ = task_runner;
+  }
+
   ~V4LocalDatabaseManager() override;
 
   // The callback called each time the protocol manager downloads updates