[WebCrypto] Cleanup in LoadTestKeys() in ecdh unit tests

content/test/data/webcrypto/ecdh.json

 // TODO(eroman): Use known test vectors. The samples I used here I generated myself.
 [
   // Test key derivation using ECDH (P-256), using a public and private key from
   // different key pairs.
   {
     "public_key": {
       "crv": "P-256",
       "kty":"EC",
       "x":"u6dWhaRHrvFF0CiFVuUUFafU6ixveQf9trHDXJ8hgV8",
       "y":"thSm4LIY35vDD-5LE454eB7TShn919DVVGZ_7tWdjTE"
@@ skipping 131 matching lines @@
     },
 
     "public_key": {
       "crv":"P-521",
       "kty":"EC",
       "x":"ADRllQ0B7icrnJ7ib2r-CXvymGFiC_3f6_o0SzLMBIggM8ndQm9l768SToMy1hUo64JsofGSQ37P4CRqT_QeivBD",
       "y":"ALKEzew1Xe4Sv86lZVqb2xxZ0l7WrE3DPJ93fUtSPih5iH8jg0GPDKMVoA5ffFmqPwbdgS2BK18PBFIT7QDGb2Zx"
     },
 
     "length_bits": 521,
-    "derived_bytes": "0117D54D84379D0FD385BE068455A77A5366AB534FF172AB0A121F37D180DCCD19607ABB0C41CB9F6F12B01303AC4A69DC2D1D05180181FD496D9769B46BFFEC3400"
+    "derived_bytes": "0117D54D84379D0FD385BE068455A77A5366AB534FF172AB0A121F37D180DCCD19607ABB0C41CB9F6F12B01303AC4A69DC2D1D05180181FD496D9769B46BFFEC3400",
+    "valid_p521_keys": true // Special variable use by ecdh_unittest.cc::LoadTestKeys
   },
 
   // Same as above but with the public/private switched.
   {
     "public_key": {
       "crv":"P-521",
       "kty":"EC",
       "x":"ACw6DX7wqwHVO-JzyOet0B-r10YVLv5R5q_IfiWCzclg0u_x57NCtOcFCFpM2ZnS22tyYjZb0gBHGcgUE_I-h-6s",
       "y":"Actm2tCHBPOKLZMpJV3DaVOluln9zBsE2I0g6iV73I4M-liqA1rLSJN8q-vcSQtZF0JvzwuvGkGuTbvT_DaRQ2pf"
     },
@@ skipping 241 matching lines @@
       "d":"uN2YSQvxuxhQQ9Y1XXjYi1vr2ZTdzuoDX18PYu4LU-0",
       "x":"S2S3tjygMB0DkM-N9jYUgGLt_9_H6km5P9V6V_KS4_4",
       "y":"03j8Tyqgrc4R4FAUV2C7-im96yMmfmO_5Om6Kr8YP3o",
       "use": "enc"
     },
 
     "private_key_error": "DataError: The JWK \"use\" member was inconsistent with that specified by the Web Crypto call. The JWK usage must be a superset of those requested"
   }
 ]