[webcrypto] Support 256-bit AES-GCM for OpenSSL implementation.

content/child/webcrypto/openssl/aes_gcm_openssl.cc

Index: content/child/webcrypto/openssl/aes_gcm_openssl.cc
diff --git a/content/child/webcrypto/openssl/aes_gcm_openssl.cc b/content/child/webcrypto/openssl/aes_gcm_openssl.cc
index 0bc927bafb161498be7844a9013f9be6b0ed34b7..bcb13642ba993f2b296bb572fd1908a0f3609b35 100644
--- a/content/child/webcrypto/openssl/aes_gcm_openssl.cc
+++ b/content/child/webcrypto/openssl/aes_gcm_openssl.cc
@@ -26,7 +26,8 @@ const EVP_AEAD* GetAesGcmAlgorithmFromKeySize(unsigned int key_size_bytes) {
   switch (key_size_bytes) {
     case 16:
       return EVP_aead_aes_128_gcm();
-    // TODO(eroman): Hook up 256-bit support when it is available.
+    case 32:
+      return EVP_aead_aes_256_gcm();
     default:
       return NULL;
   }