Fix #4071: Don't share canonical URL unless base domains differ

[kylehickinson]

Summary of Changes

This pull request fixes #4071

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

• Visit youtube.com directly, then visit a video from the home page. Verify that Share With… > Copy shares the watch URL
• Go to google.com and search for something that would result in news articles appearing. Switch to the "News" tab results and select an AMP page. Verify that Share With… > Copy shares the underlying news article URL and not the Google AMP url

Screenshots

YouTube	AMP

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).