fix(WebSocket): don't patch EventTarget methods twice

[tlancina]

On older versions of Chrome, patching via property descriptors isn't available, but unlike Safari, window.EventTarget exists and is already patched, so patching EventTarget methods on WebSocket.prototype again causes them to call the first patched method which then infinitely calls itself.

I didn't provide a test because the tests fail on any older Chrome version (not sure the exact range, tested on 39 and 41). I don't have access to SauceLabs, but I believe #192 will cause failure on at least the Android 4.4 case.

This is fairly high priority for Ionic as it seems to crash Android on some devices when using livereload.

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[tlancina]

I signed it ✏️

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[mhevery]

@vicb Could you merge this and cut a new Zone release. This is blocking Ionic.

[tlancina]

Thanks!