Risk of overwriting networkID of a transaction

**Describe the bug**

MyEtherWallet.com (MEW) and MetaMask don't alert when they are on different network.

I wanted to [deploy a contract with bytecode](https://www.myetherwallet.com/#contracts) on MEW to the main net. I was on the main net according to the dropdown on the upper right of MEW, and MEW also asked me to confirm the deployment on *ETH main net*. It shows that the `networkID` shall be defined in the transaction.
However, my MetaMask account was initially on *Ropsten test net*. When confirming the transaction, it stays on Ropsten and only asked for tx confirmation. No warning/alert from either side. It's a risk for users of messing up network.
Eventually, the contract was deployed on Ropsten, therefore implying a risk of overwriting the networkID of a transaction and reduces the usability for users.

**To Reproduce**
1. Go to [MEW](https://www.myetherwallet.com/) and select Network 'ETH' of the upper right dropdown
2. Go to 'Contract' section and select 'Deploy Contract'
3. Paste the bytecode of a sample smart contract, enter gasLimit, select 'MetaMask/Mist' and "Connect to MetaMask"
4. Click on 'Sign Transaction' and you can see in the 'Raw Transaction' and 'Signed Transaction', the `chainId = 1`.
5. Make sure that MetaMask is on Ropsten test net. (This can be switched/set at anytime from step 1 till step 4)
6. Click on 'Deploy Contract' and confirm that 'Yes, I'm sure! Make transaction ' (to ETH main net)
7. Confirm tx on MetaMask
8. Track the tx and the deployed contract. They are on Ropsten test net instead of main net.

**Expected behavior**

MetaMask shall follow the transaction prepared by the application and deploy the contract on main net, even though the user forgot to switch the network.

**Browser details:**
- OS: macOS 10.14
- Browser: Chrome
- MetaMask Version: 5.3.1

Start by reproducing the MEW and MetaMask network mismatch using the listed deployment steps, especially the chainId 1 transaction sent while MetaMask is on Ropsten. Determine how the extension handles the selected network during confirmation; done means the user is warned or the transaction cannot be confirmed on the wrong network.

Written by the indexing model from the issue text.