Define global web3 as non-enumerable

[Gudahtt]

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.

[metamaskbot]

Builds ready [c447454]

• builds: chrome, firefox, opera
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• dep viz: background
• all artifacts

Page Load Metrics (591 ± 48 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	31	93	41	15	7
		domContentLoaded	349	728	590	101	48
		load	351	729	591	101	48
		domInteractive	349	728	589	101	49

[Gudahtt]

We'll be stopping the web3 injection soon hopefully (see #7163), but this should suffice in the meantime.