[WebCrypto] Throw syntaxError if keyUsage is empty in ImportKey

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 329 matching lines @@
       "kty":"EC",
       "crv":"P-256",
       "x":"S2S3tjygMB0DkM-N9jYUgGLt_9_H6km5P9V6V_KS4_4",
       "y":"03j8Tyqgrc4R4FAUV2C7-im96yMmfmO_5Om6Kr8YP3o"
     },
 
     "length_bits": 264,
     "error": "OperationError: Length specified for ECDH key derivation is too large. Maximum allowed is 256 bits"
   },
 
-  // The public key parameter is in fact a private key

[eroman, 2014/12/17 21:17:42]

This test should not be removed.

If it is failing with this change, you must instead change the test expectation.

You will probably need to modify the test harness and introduce a
"public_key_error" field.

Search for "private_key_error" to see what I mean.

Once this has been addressed, the rest of the change looks good.

[eroman, 2014/12/17 21:21:03]

Actually I am OK with removing this test, as we have an equivalent one  in:

https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

LGTM then

-  // (not allowed by web crypto), even though it could in practice work.
-  {
-    "private_key": {
-      "kty":"EC",
-      "crv":"P-256",
-      "x":"u6dWhaRHrvFF0CiFVuUUFafU6ixveQf9trHDXJ8hgV8",
-      "y":"thSm4LIY35vDD-5LE454eB7TShn919DVVGZ_7tWdjTE",
-      "d":"CQ8uF_-zB1NftLO6ytwKM3Cnuol64PQw5qOuCzQJeFU"
-    },
-    "public_key": {
-      "kty":"EC",
-      "crv":"P-256",
-      "d":"uN2YSQvxuxhQQ9Y1XXjYi1vr2ZTdzuoDX18PYu4LU-0",
-      "x":"S2S3tjygMB0DkM-N9jYUgGLt_9_H6km5P9V6V_KS4_4",
-      "y":"03j8Tyqgrc4R4FAUV2C7-im96yMmfmO_5Om6Kr8YP3o"
-    },
-
-    "length_bits": 256,
-    "error": "InvalidAccess: The public parameter for ECDH key derivation is not a public EC key"
-  },
-
   // Curve mismatch (public key is for P-521 however private key was for P-256).
   {
     "private_key": {
       "kty":"EC",
       "crv":"P-256",
       "x":"u6dWhaRHrvFF0CiFVuUUFafU6ixveQf9trHDXJ8hgV8",
       "y":"thSm4LIY35vDD-5LE454eB7TShn919DVVGZ_7tWdjTE",
       "d":"CQ8uF_-zB1NftLO6ytwKM3Cnuol64PQw5qOuCzQJeFU"
     },
 
@@ skipping 44 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"
   }
 ]