feat(wallet): disable EIP-1559 on Polygon and Avalanche

Our current EIP-1559 gas oracle queries the Etherscan API, which only
supports Ethereum mainnet. As a result, priority fee estimations on
custom networks use results for Ethereum mainnet, which can either
result in transactions being stuck in the mempool forever, or in
over-payment of gas fees.

This commit will be reverted in brave/brave-browser#20469.

components/brave_wallet/browser/json_rpc_service.cc

@@ -311,6 +311,21 @@ void JsonRpcService::UpdateIsEip1559(const std::string& chain_id,
       if (!id || *id != chain_id)
         continue;
 
+      // Disable EIP-1559 on selected networks.
+      //
+      // This list needs to be updated until we have a generic EIP-1559 gas
+      // oracle. See: https://github.com/brave/brave-browser/issues/20469.
+      if (chain_id == "0x13881" ||  // Polygon Mumbai Testnet
+          chain_id == "0x89" ||     // Polygon Mainnet
+          chain_id == "0xa86a" ||   // Avalanche Mainnet
+          chain_id == "0xa869"      // Avalanche Fuji Testnet
+      ) {
+        changed =
+            custom_network.FindBoolKey("is_eip1559").value_or(false) != false;
+        custom_network.SetBoolKey("is_eip1559", false);
+        break;
+      }
+
       changed = custom_network.FindBoolKey("is_eip1559").value_or(false) !=
                 is_eip1559;
       custom_network.SetBoolKey("is_eip1559", is_eip1559);