Estimate the overhead of an X509 object without sizeof

To unblock optimizations in hopes of reducing the overhead, we're
planning to make X509 opaque in BoringSSL. This aligns with upstream
OpenSSL, which has made most of these structs opaque.

Instead, use an estimate. The sizeof-based estimate was undercounting
the overhead anyway because there are many structures underneath X509,
some of which are already opaque. I just rounded sizeof(X509) +
sizeof(X509_CINF) up.

(Even this is likely still undercounting it because X509 objects are
very malloc-heavy and duplicate large chunks of the certificate. They're
not a very efficient representation and can't be made efficient without
first hiding the structs. In Chrome, we stopped using them altogether
and just retain the byte string, parsing as needed.)

TEST=ci

Change-Id: Icdc729ceba7eadf002bec5e080fc3e0adf7c4b56
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209920
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>
Auto-Submit: David Benjamin <[email protected]>

Author: davidben

runtime/bin/security_context.cc

@@ -122,7 +122,11 @@ static void ReleaseCertificate(void* isolate_data, void* context_pointer) {
 
 static intptr_t EstimateX509Size(X509* certificate) {
   intptr_t length = i2d_X509(certificate, NULL);
-  return length > 0 ? length : 0;
+  length = length > 0 ? length : 0;
+  // An X509 is a tree of structures, which are either opaque or will be opaque
+  // in the future. Estimate the overhead to 512 bytes by rounding up
+  // sizeof(X509) + sizeof(X509_CINF).
+  return length + 512;
 }
 
 // Returns the handle for a Dart object wrapping the X509 certificate object.
@@ -154,7 +158,7 @@ Dart_Handle X509Helper::WrappedX509Certificate(X509* certificate) {
     return status;
   }
   const intptr_t approximate_size_of_certificate =
-      sizeof(*certificate) + EstimateX509Size(certificate);
+      EstimateX509Size(certificate);
   ASSERT(approximate_size_of_certificate > 0);
   Dart_NewFinalizableHandle(result, reinterpret_cast<void*>(certificate),
                             approximate_size_of_certificate,