Use {To/From}JavaTime for policy timestamps

components/policy/proto/device_management_backend.proto

Index: components/policy/proto/device_management_backend.proto
diff --git a/components/policy/proto/device_management_backend.proto b/components/policy/proto/device_management_backend.proto
index 46bd26188c9c005642044a0b61c4ea6433324c56..93f0ac568faba9e023f06e7481da62af442a6146 100644
--- a/components/policy/proto/device_management_backend.proto
+++ b/components/policy/proto/device_management_backend.proto
@@ -201,7 +201,9 @@ message PolicyFetchRequest {
   //   google/ios/user => ChromeSettingsProto
   optional string policy_type = 1;
 
-  // This is the last policy timestamp that client received from server.
+  // This is the last policy timestamp that client received from server. The
+  // expectation is that this field is filled by the value of
+  // PolicyData.timestamp from the last policy received by the client.
   optional int64 timestamp = 2;
 
   // Tell server what kind of security signature is required.
@@ -285,11 +287,11 @@ message PolicyData {
   // See PolicyFetchRequest.policy_type.
   optional string policy_type = 1;
 
-  // [timestamp] is milliseconds since Epoch in UTC timezone. It is
-  // included here so that the time at which the server issued this
-  // response cannot be faked (as protection against replay attacks).
-  // It is the timestamp generated by DMServer, NOT the time admin
-  // last updated the policy or anything like that.
+  // [timestamp] is milliseconds since Epoch in UTC timezone (Java time). It is
+  // included here so that the time at which the server issued this response
+  // cannot be faked (as protection against replay attacks). It is the timestamp
+  // generated by DMServer, NOT the time admin last updated the policy or
+  // anything like that.
   optional int64 timestamp = 2;
 
   // The DM token that was used by the client in the HTTP POST header
@@ -536,7 +538,7 @@ message DevicePolicyResponse {
 }
 
 message TimePeriod {
-  // [timestamp] is milli seconds since Epoch in UTC timezone.
+  // [timestamp] is milliseconds since Epoch in UTC timezone (Java time).
   optional int64 start_timestamp = 1;
   optional int64 end_timestamp = 2;
 }
@@ -1023,8 +1025,9 @@ message RemoteCommandResult {
   // RemoteCommand protobuf that contained the command.
   optional int64 command_id = 2;
 
-  // The time at which the command was executed, if the the result is
-  // RESULT_SUCCESS.
+  // The timestamp representing time at which the command was executed, if the
+  // result is RESULT_SUCCESS. The timestamp is milliseconds since Epoch in UTC
+  // timezone (Java time).
   optional int64 timestamp = 3;
 
   // Extra information sent to server as result of execution, expected to be a