Event handlers causing memory leak in inpage.js

[gkaemmer]

Hello! I just found the cause of an issue that's been bugging me for a while.

Describe the bug
There's a bug in inpage.js at line 35. Repeated calls to isApproved or isUnlocked add event listeners on window that are never removed. To properly remove event listeners, removeEventListener must be called with the exact reference to the original handler, which is not the case here.

To Reproduce
With metamask installed, open chrome devtools in a new window. Then:

getEventListeners(window).message.length
// => 3
await ethereum._metamask.isApproved()
// => false (or true, doesn't matter)
getEventListeners(window).message.length
// => 4
await ethereum._metamask.isApproved()
await ethereum._metamask.isApproved()
getEventListeners(window).message.length
// => 6

Expected behavior
Events should be cleaned up.

Screenshots
N/A

Browser details (please complete the following information):
I assume anywhere that inpage.js runs is affected by this, but I'm using the latest version of chrome and metamask 6.0.1.

Additional context
This causes memory and CPU to go off the charts in windows that have called isUnlocked or isApproved a bunch of times, which is common when polling for wallet status. Also I think there are a number of similar leaks in inpage.js--the onMessage function never successfully removes listeners.

It should be pretty trivial to fix this--let me know if you need help with it.