fix iframe placeholder html

[kuceb]

• fix dom snapshot iframe placeholder HTML not rendering #5295

User facing changelog

Bug Fix:

• fixed DOM snapshots failing to render iframe placeholder content

Additional details

• dom snapshot iframes were not correctly rendering our placeholder content, so you would see blank iframe.
• switched to using base64 encoding, which seems to fix the encoding problem
• we can't directly query the iframe contents in tests, so this would be a good candidate for visual regression. However, for now I have added test for correct base64 encoding

How has the user experience changed?

• when vieing dom snapshot:
  before:
  
  after:
  

PR Tasks

• Have tests been added/updated?
• N/A Has a PR for user-facing changes been opened in cypress-documentation?
• N/A Have API changes been updated in the type definitions?
• N/A Have new configuration options been added to the cypress.schema.json?

[kuceb]

strange gyp failure during build, reran ci

[cypress]

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Test summary

3371 • 0 • 47 • 0

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Run details

Project	cypress
Status	Passed
Commit	19c6e76
Started	Oct 4, 2019 8:31 PM
Ended	Oct 4, 2019 8:36 PM
Duration	05:13 💡
OS	Linux Debian - 9.8
Browser	Multiple

View run in Cypress Dashboard ➡️

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[chrisbreiding]

Manually tested this and it looks good 👍

[jennifer-shehane]

@bkucera Test is failing saying cannot get getBoundingClientRect of undefined here https://github.com/cypress-io/cypress/blob/iframe-placeholder-fix/packages/driver/test/cypress/integration/e2e/domSnapshots.spec.js#L43

[jennifer-shehane]

Actually, this is also failing in another PR - is this broken in develop? #5272

.