Added Curve25519-donna changes.

Added a wrapper class that implements the following API calls which for
Curve25519.
+ ScalarMult to compute the shared key.
+ ScalarBaseMult to get public key.
+ ConvertToPrivateKey returns a private key from random bytes.

Per agl/wtc, grabbed the rev 234205ff from the git repo
(https://github.com/agl/curve25519-donna/tree/234205ff1ecaf6b3c1dc76798a462c4293f31fdb)
and checked it in to crypto/ because that version has pure Google copyright.

R=wtc@chromium.org,agl@chromium.org,rsleevi@chromium.org
TEST=crypto unit tests

Review URL: https://chromiumcodereview.appspot.com/12457004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@187074 0039d316-1c4b-4281-b951-d872f2087c98

crypto/crypto.gyp

@@ -186,6 +186,9 @@
         'crypto_module_blocking_password_delegate.h',
         'cssm_init.cc',
         'cssm_init.h',
+        'curve25519.cc',
+        'curve25519.h',
+        'curve25519-donna.c',
         'ghash.cc',
         'ghash.h',
         'ec_private_key.h',
@@ -262,6 +265,7 @@
         'run_all_unittests.cc',
 
         # Tests.
+        'curve25519_unittest.cc',
         'ec_private_key_unittest.cc',
         'ec_signature_creator_unittest.cc',
         'encryptor_unittest.cc',