style(ctb): Rename _isSystemMessageSender to _isOtherMessenger (#3022)

packages/contracts-bedrock/contracts/L1/L1CrossDomainMessenger.sol

@@ -46,7 +46,7 @@ contract L1CrossDomainMessenger is CrossDomainMessenger, Semver {
      *
      * @return True if the message was sent from the messenger, false otherwise.
      */
-    function _isSystemMessageSender() internal view override returns (bool) {
+    function _isOtherMessenger() internal view override returns (bool) {
         return msg.sender == address(portal) && portal.l2Sender() == otherMessenger;
     }
 

packages/contracts-bedrock/contracts/L2/L2CrossDomainMessenger.sol

@@ -52,7 +52,7 @@ contract L2CrossDomainMessenger is CrossDomainMessenger, Semver {
      *
      * @return True if the message sender is the L1CrossDomainMessenger on L1.
      */
-    function _isSystemMessageSender() internal view override returns (bool) {
+    function _isOtherMessenger() internal view override returns (bool) {
         return AddressAliasHelper.undoL1ToL2Alias(msg.sender) == otherMessenger;
     }
 

packages/contracts-bedrock/contracts/universal/CrossDomainMessenger.sol

@@ -274,7 +274,7 @@ abstract contract CrossDomainMessenger is
             _message
         );
 
-        if (_isSystemMessageSender()) {
+        if (_isOtherMessenger()) {
             // Should never happen.
             require(msg.value == _value, "Mismatched message value.");
         } else {
@@ -349,7 +349,7 @@ abstract contract CrossDomainMessenger is
      *         contracts because the logic for this depends on the network where the messenger is
      *         being deployed.
      */
-    function _isSystemMessageSender() internal view virtual returns (bool);
+    function _isOtherMessenger() internal view virtual returns (bool);
 
     /**
      * @notice Sends a low-level message to the other messenger. Needs to be implemented by child