contracts-bedrock: L2CrossDomainMessenger legibility (#3266)

The `initialize()` function in the L2CrossDomainMessenger
sets some blocked system addresses. It was using `address(this)`
but in reality the contract is a predeploy at a specific address.
It should not be deployed, the `initialize` function will never
run. For the unit tests, the contracts are placed at their correct
addresses. This makes the testing environment as close as possible
to what is experienced on a live network.

This PR hardcodes in the L2 cross domain messenger constant
from the predeploys library instead of using `address(this)`.
In practice this value is set using the chain ops tooling,
so this makes it more clear as to what the value should be.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/ninety-experts-wave.md

+---
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Fix legibility in the L2CrossDomainMessengerInitializer

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

@@ -32,7 +32,7 @@ contract L2CrossDomainMessenger is CrossDomainMessenger, Semver {
      */
     function initialize(address _l1CrossDomainMessenger) public initializer {
         address[] memory blockedSystemAddresses = new address[](2);
-        blockedSystemAddresses[0] = address(this);
+        blockedSystemAddresses[0] = Predeploys.L2_CROSS_DOMAIN_MESSENGER;
         blockedSystemAddresses[1] = Predeploys.L2_TO_L1_MESSAGE_PASSER;
         __CrossDomainMessenger_init(_l1CrossDomainMessenger, blockedSystemAddresses);
     }