Explicitly initialize pointer in SafeBrowsingPrivateEventRouter

The binary_upload_service_ pointer can sometimes hold uninitialized
data, which leads to calling IsAuthorized() on an invalid object. In
my case this was due to a turned-off feature flag.

Change-Id: I1db50db3350c43f48b45b0b7a310ac83608147b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884994
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Bettina Dea <bdea@chromium.org>
Commit-Queue: Bettina Dea <bdea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710058}

chrome/browser/extensions/api/safe_browsing_private/safe_browsing_private_event_router.h

@@ -204,7 +204,7 @@ class SafeBrowsingPrivateEventRouter : public KeyedService {
   content::BrowserContext* context_;
   signin::IdentityManager* identity_manager_ = nullptr;
   EventRouter* event_router_ = nullptr;
-  safe_browsing::BinaryUploadService* binary_upload_service_;
+  safe_browsing::BinaryUploadService* binary_upload_service_ = nullptr;
   std::unique_ptr<policy::CloudPolicyClient> client_;
   PrefChangeRegistrar registrar_;