Relax DCHECKs regarding Threads and RefCounts in iOS' HttpServer.

ios/web/public/test/http_server.mm

Index: ios/web/public/test/http_server.mm
diff --git a/ios/web/public/test/http_server.mm b/ios/web/public/test/http_server.mm
index c5217160571ec5b3fd3ce695aa0c8500d613e35a..c20f7d1dd586bcc90d78a5ecd3051f3a2887e647 100644
--- a/ios/web/public/test/http_server.mm
+++ b/ios/web/public/test/http_server.mm
@@ -80,6 +80,12 @@ void HttpServer::InitHttpServer() {
   // Note: This block is called from an arbitrary GCD thread.
   id process_request =
       ^GCDWebServerResponse*(GCDWebServerDataRequest* request) {
+
+      // TODO(crbug.com/707764): Verify that the relaxation of the cross-thread

[sdefresne, 2017/04/03 14:18:16]

I think that a correct fix would be to:

1. split GetResponseProviderForRequest in two
  - GetResponseProviderForRequestImpl that does all the work except the locking,
  - GetResponseProviderForRequest that take the lock then call
GetResponseProviderForRequestImpl
2. changing here to acquire the lock, then call
GetResponseProviderForRequestImpl
3. changing the comment to the following

  // Relax the cross-thread access restriction to non-thread-safe RefCount.
  // |provider_list_lock_| protects the access to the
  // RefCountedResponseProviderWrapper instances.

+      // access restriction is correct.
+      base::ScopedAllowCrossThreadRefCountAccess
+          allow_cross_thread_ref_count_access;
+
       ResponseProvider::Request provider_request =
           ResponseProviderRequestFromGCDWebServerRequest(request);
       scoped_refptr<RefCountedResponseProviderWrapper>
@@ -225,6 +231,10 @@ void HttpServer::RemoveResponseProvider(ResponseProvider* response_provider) {
 
 void HttpServer::RemoveAllResponseProviders() {
   DCHECK([NSThread isMainThread]);
+  // TODO(crbug.com/707764): Verify that the relaxation of the cross-thread

[sdefresne, 2017/04/03 14:18:16]

I think you can change this to:

  // Relax the cross-thread access restriction to non-thread-safe RefCount.
  // |provider_list_lock_| protects the access to the
  // RefCountedResponseProviderWrapper instances.

+  // access restriction is correct.
+  base::ScopedAllowCrossThreadRefCountAccess
+      allow_cross_thread_ref_count_access;
   base::AutoLock autolock(provider_list_lock_);
   providers_.clear();
 }