WebCrypto: Add LayoutTests for ECDSA.

LayoutTests/crypto/ecdsa-generate-key-expected.txt

Index: LayoutTests/crypto/ecdsa-generate-key-expected.txt
diff --git a/LayoutTests/crypto/ecdsa-generate-key-expected.txt b/LayoutTests/crypto/ecdsa-generate-key-expected.txt
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+Test generating an EC key pair for ECDSA.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+Generating a key pair...
+PASS keyPair.toString() is '[object Object]'
+PASS keyPair.publicKey.type is 'public'
+PASS keyPair.publicKey.algorithm.name is 'ECDSA'
+PASS keyPair.publicKey.algorithm.namedCurve is 'P-256'
+PASS keyPair.publicKey.usages is ["verify"]
+PASS keyPair.privateKey.type is 'private'
+PASS keyPair.privateKey.algorithm.name is 'ECDSA'
+PASS keyPair.privateKey.algorithm.namedCurve is 'P-256'
+PASS keyPair.privateKey.usages is ["sign"]
+
+Testing that custom attributes on keys survive garbage collection...
+PASS keyPair.publicKey.foo is 'bar'
+PASS successfullyParsed is true
+
+TEST COMPLETE
+