Define global `web3` as non-enumerable (#8634)

We inject `web3` globally on most websites. This has been breaking
websites that attempted to serialize the `window` object, because any
attempt to access certain `web3` properties (such as `web3.eth.mining`)
would throw an error. This is because `web3` defined a getter for these
properties that would call `.send([method])`, which doesn't work for
most methods.

An example of a site that this breaks is `Storybook`, when the
`@storybook/addon-actions` addon is being used. When using storybook
with this addon and with the MetaMask extension installed, actions
would not be properly dispatched because an error would be thrown in
the attempt to serialize the event (which includes a reference to the
`window`).

The `web3` global we inject is now defined as non-enumerable, so it
will be skipped automatically in any attempt to serialize the `window`
object.

app/scripts/inpage.js

@@ -79,7 +79,12 @@ web3.setProvider = function () {
 }
 log.debug('MetaMask - injected web3')
 
-window.ethereum._web3Ref = web3.eth
+Object.defineProperty(window.ethereum, '_web3Ref', {
+  enumerable: false,
+  writable: true,
+  configurable: true,
+  value: web3.eth,
+})
 
 // setup dapp auto reload AND proxy web3
 setupDappAutoReload(web3, window.ethereum._publicConfigStore)

app/scripts/lib/auto-reload.js

@@ -8,7 +8,7 @@ export default function setupDappAutoReload (web3, observable) {
   let lastSeenNetwork
   let hasBeenWarned = false
 
-  global.web3 = new Proxy(web3, {
+  const web3Proxy = new Proxy(web3, {
     get: (_web3, key) => {
       // get the time of use
       lastTimeUsed = Date.now()
@@ -26,6 +26,13 @@ export default function setupDappAutoReload (web3, observable) {
     },
   })
 
+  Object.defineProperty(global, 'web3', {
+    enumerable: false,
+    writable: true,
+    configurable: true,
+    value: web3Proxy,
+  })
+
   observable.subscribe(function (state) {
     // if the auto refresh on network change is false do not
     // do anything