immutableData.getIn first arg should be an array

[bbondy]

Fix #10415

Auditors: @bsclifton

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).
  No but it's kind of a lame introduced bug that is not likely to happen again now that we're on immutable.
• 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]

Changes look good! Would be hard to test

I scanned for other possible immutable issues- the only one I found was this (cc: @NejcZdovc)

browser-laptop/test/unit/app/browser/reducers/bookmarkFoldersReducerTest.js

Line 152 in 1c249e6

.set(['bookmarkFolders'], Immutable.fromJS({

(this should be using like .set('key') or .setIn(['key']))