Refactor how "length" parameter is obtained for HmacKeyParams.

Source/modules/crypto/HmacKeyParams.cpp

Index: Source/modules/crypto/HmacKeyParams.cpp
diff --git a/Source/modules/crypto/HmacKeyParams.cpp b/Source/modules/crypto/HmacKeyParams.cpp
index 9e58ad45180b82ef285649eb2fdcc25d8fd1b2d4..1b918bdd0c6f3517f985a51ae148f9fd77091a4c 100644
--- a/Source/modules/crypto/HmacKeyParams.cpp
+++ b/Source/modules/crypto/HmacKeyParams.cpp
@@ -44,8 +44,9 @@ Algorithm* HmacKeyParams::hash()
 
 unsigned HmacKeyParams::length(bool& isNull)
 {
-    isNull = !m_algorithm.hmacKeyParams()->hasLength();
-    return isNull ? 0 : m_algorithm.hmacKeyParams()->length();
+    unsigned length;
+    isNull = !m_algorithm.hmacKeyParams()->getLength(length);
+    return length;
 }
 
 HmacKeyParams::HmacKeyParams(const WebKit::WebCryptoAlgorithm& algorithm)