fix getEventSourceWrapper window on iframe

[minimit]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

When using iframes HMR was initializing inside each iframe, this way intead we use the parent instance of window.__whmEventSourceWrapper so you have no duplicate listeners and no limit on how many iframes you can have before the HMR hungs up with too many calls.

Breaking Changes

None

Additional Info

[codecov]

Codecov Report

Merging #358 (9305296) into master (fe33d59) will decrease coverage by 0.24%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
- Coverage   88.63%   88.38%   -0.25%     
==========================================
  Files           3        3              
  Lines         264      267       +3     
  Branches       76       77       +1     
==========================================
+ Hits          234      236       +2     
- Misses         30       31       +1     

Impacted Files	Coverage Δ
client.js	83.92% <87.50%> (-0.32%)	⬇️

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[minimit]

I've tested with iframes and it works good, you can have as many iframes that you want and the hot reloads styles inside and outside the iframes!

[minimit]

Closing for revision in firefox

[gladziu90]

I think we should not keep it cached now. e.g. On this reload server is closing web socket connection
module.hot.accept() => { window.location.reload(); });
while when creating new EventSourceWrapper(); it work properly.