don’t convert paths to immutable

[bridiver]

Converting to immutable is slow

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

This change is a little weird, because getTabInternalIndexByTabId (which is called inside getPathByTabId) returns an Immutable object. So the caller would need to know if the return value is mutable or not (ex: check for return.toJS etc). Let me check in a test which shows the problem

[bsclifton]

Disregard previous comment- I was pulling up code locally and had unintentionally opened frameState => getPathByTabId 😄 This change looks good!

[bsclifton]

Added unit test; ++