contracts-bedrock: prevent deposits with too small gaslimit

The deposits sent to L2 should have a gas limit that at least
covers the intrinsic gas. If the deposits do not pay that much,
then it is possible for cause resource usage on L2 without
paying for it. This is not a critical bug because the user
would still need to pay for L1 inclusion.

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

@@ -371,6 +371,9 @@ contract OptimismPortal is Initializable, ResourceMetering, Semver {
             );
         }
 
+        // Prevent depositing transactions that have too small of a gas limit.
+        require(_gasLimit >= 21_000, "OptimismPortal: gas limit must cover instrinsic gas cost");
+
         // Transform the from-address to its alias if the caller is a contract.
         address from = msg.sender;
         if (msg.sender != tx.origin) {

packages/contracts-bedrock/contracts/test/CrossDomainOwnable.t.sol

@@ -52,13 +52,13 @@ contract CrossDomainOwnableThroughPortal_Test is Portal_Initializer {
         vm.recordLogs();
 
         vm.prank(alice);
-        op.depositTransaction(
-            address(setter),
-            0,
-            10000,
-            false,
-            abi.encodeWithSelector(XDomainSetter.set.selector, 1)
-        );
+        op.depositTransaction({
+            _to: address(setter),
+            _value: 0,
+            _gasLimit: 21_000,
+            _isCreation: false,
+            _data: abi.encodeWithSelector(XDomainSetter.set.selector, 1)
+        });
 
         // Simulate the operation of the `op-node` by parsing data
         // from logs

packages/contracts-bedrock/contracts/test/OptimismPortal.t.sol

@@ -37,6 +37,21 @@ contract OptimismPortal_Test is Portal_Initializer {
         op.depositTransaction(address(1), 1, 0, true, hex"");
     }
 
+    /**
+     * @notice Prevent gasless deposits from being force processed in L2 by
+     *         ensuring that they have a large enough gas limit set.
+     */
+    function test_depositTransaction_smallGasLimit_reverts() external {
+        vm.expectRevert("OptimismPortal: gas limit must cover instrinsic gas cost");
+        op.depositTransaction({
+            _to: address(1),
+            _value: 0,
+            _gasLimit: 0,
+            _isCreation: false,
+            _data: hex""
+        });
+    }
+
     // Test: depositTransaction should emit the correct log when an EOA deposits a tx with 0 value
     function test_depositTransaction_noValueEOA_succeeds() external {
         // EOA emulation