Adding the Copresence RpcHandler and HttpPost helper.

components/copresence/proto/identity.proto

 syntax = "proto2";
 package copresence;
 option optimize_for = LITE_RUNTIME;
 import "enums.proto";
 message Identity {
   optional IdentityType type = 1;
-  optional string obfuscated_gaia_id = 2 [deprecated = true];
   optional string chromecast_id = 3;
   optional string android_id = 4;
   optional string chrome_id = 5;
   optional int64 gaia_id = 6;
 }
 message DeviceIdentity {
   optional Identity identity = 1;
   optional DeviceVersion version = 2;
 }
 message DeviceVersion {
   optional string brand = 1;
   optional string product = 2;
   optional string version = 3;
 }