[Web Payment] Two phase shutdown for PaymentAppService.

Before this patch, PaymentAppService would release its references in the
destructor, which is the second phase of browser shutdown, as opposed to
the KeyedService::Shutdown() method, which is the first phase, where all
references should be released. This may have caused a crash when closing
the browser.

This is a speculative patch to have PaymentAppService release its
references to the payment app factories during the first phase of
browser shutdown.

After this patch, PaymentAppService releases its references in the
KeyedService::Shutdown() method, which is the first phase of browser
shutdown.

Bug: 1077713
Change-Id: Ic5785f703156bea2af7a1a03fca4ca6a39bb7dfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2236464Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776561}

components/payments/content/payment_app_service.cc

@@ -32,4 +32,8 @@ void PaymentAppService::Create(
   }
 }
 
+void PaymentAppService::Shutdown() {
+  factories_.clear();
+}
+
 }  // namespace payments

components/payments/content/payment_app_service.h

@@ -30,6 +30,9 @@ class PaymentAppService : public KeyedService {
   // Create payment apps for |delegate|.
   void Create(base::WeakPtr<PaymentAppFactory::Delegate> delegate);
 
+  // KeyedService implementation:
+  void Shutdown() override;
+
  private:
   std::vector<std::unique_ptr<PaymentAppFactory>> factories_;