WebCrypto: Add interface for exportKey().

Source/testing/runner/MockWebCrypto.cpp

Index: Source/testing/runner/MockWebCrypto.cpp
diff --git a/Source/testing/runner/MockWebCrypto.cpp b/Source/testing/runner/MockWebCrypto.cpp
index 48e9a2209f7058e2ca53d6f47d613aa98d7f0a54..a0b48af7567f11da428ec0e56b098f12fd5f2f28 100644
--- a/Source/testing/runner/MockWebCrypto.cpp
+++ b/Source/testing/runner/MockWebCrypto.cpp
@@ -125,4 +125,17 @@ void MockWebCrypto::importKey(WebKit::WebCryptoKeyFormat, const unsigned char* k
     result.completeWithKey(WebKit::WebCryptoKey::create(0, type, extractable, algorithm, usages));
 }
 
+void MockWebCrypto::exportKey(WebKit::WebCryptoKeyFormat format, const WebKit::WebCryptoKey& key, WebKit::WebCryptoResult result)
+{
+    std::string buffer;
+
+    if (format == WebKit::WebCryptoKeyFormatRaw)
+        buffer = "raw";
+    else if (format == WebKit::WebCryptoKeyFormatPkcs8)
+        buffer = "pkcs8";
+
+    result.completeWithBuffer(buffer.data(), buffer.size());
+}
+
+
 } // namespace WebTestRunner