[webcrypto] Add RSA public key SPKI import/export for NSS.

content/renderer/webcrypto/webcrypto_impl.cc

Index: content/renderer/webcrypto/webcrypto_impl.cc
diff --git a/content/renderer/webcrypto/webcrypto_impl.cc b/content/renderer/webcrypto/webcrypto_impl.cc
index 2269030d0a3a3eaf4d1f445a42b8ed909b3f4490..4c4f3bf8d137975fb7517f17211666d05f06ef19 100644
--- a/content/renderer/webcrypto/webcrypto_impl.cc
+++ b/content/renderer/webcrypto/webcrypto_impl.cc
@@ -156,6 +156,18 @@ void WebCryptoImpl::importKey(
   result.completeWithKey(key);
 }
 
+void WebCryptoImpl::exportKey(
+    blink::WebCryptoKeyFormat format,
+    const blink::WebCryptoKey& key,
+    blink::WebCryptoResult result) {
+  blink::WebArrayBuffer buffer;
+  if (!ExportKeyInternal(format, key, &buffer)) {
+    result.completeWithError();
+    return;
+  }
+  result.completeWithBuffer(buffer);
+}
+
 void WebCryptoImpl::sign(
     const blink::WebCryptoAlgorithm& algorithm,
     const blink::WebCryptoKey& key,