[webcrypto] Add length parameter to HmacKeyAlgorithm. (blink)

Source/modules/crypto/HmacKeyAlgorithm.cpp

Index: Source/modules/crypto/HmacKeyAlgorithm.cpp
diff --git a/Source/modules/crypto/HmacKeyAlgorithm.cpp b/Source/modules/crypto/HmacKeyAlgorithm.cpp
index 5d42b817f645488637f81a2878ea7372e33f32c8..db873edee535dfe717b128d4c4d8311d2193a430 100644
--- a/Source/modules/crypto/HmacKeyAlgorithm.cpp
+++ b/Source/modules/crypto/HmacKeyAlgorithm.cpp
@@ -48,6 +48,11 @@ KeyAlgorithm* HmacKeyAlgorithm::hash()
     return m_hash.get();
 }
 
+unsigned HmacKeyAlgorithm::length()
+{
+    return m_algorithm.hmacParams()->lengthBits();
+}
+
 void HmacKeyAlgorithm::trace(Visitor* visitor)
 {
     KeyAlgorithm::trace(visitor);