Avoid a reference cycle between SSLPrivateKeyInternal and mojom::SSLPrivateKey

SSLPrivateKeyInternal owns mojom::SSLPrivateKey, and mojom::SSLPrivateKey
has a reference to SSLPrivateKeyInternal. That keeps both of them alive
until a connection error happens.

This CL removes the reference from mojom::SSLPK to SSLPKI to remove the
cycle.

Bug: 866456
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Iab0a8bcd16663520873b6b7b29d9326755d93600
Reviewed-on: https://chromium-review.googlesource.com/1148095Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578186}

services/network/url_loader.cc

@@ -231,7 +231,8 @@ class SSLPrivateKeyInternal : public net::SSLPrivateKey {
       : algorithm_perferences_(algorithm_perferences),
         ssl_private_key_(std::move(ssl_private_key)) {
     ssl_private_key_.set_connection_error_handler(
-        base::BindOnce(&SSLPrivateKeyInternal::HandleSSLPrivateKeyError, this));
+        base::BindOnce(&SSLPrivateKeyInternal::HandleSSLPrivateKeyError,
+                       base::Unretained(this)));
   }
 
   // net::SSLPrivateKey: