Bookmarks Manager folder area has scrollbar in wrong position

[bsclifton]

Test plan:

1. make about:bookmarks very wide
2. folder area should keep its size
3. make about:bookmarks very narrow
4. folder area should fill window width

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- Did you search for similar issues before submitting this one? Yes

• Describe the issue you encountered:
  When visiting about:bookmarks, if the window is large, the scrollbars are put into the wrong place

• Platform (Win7, 8, 10? macOS? Linux distro?):
  macOS

• Brave Version (revision SHA):
  0.15.3 preview 3

• Steps to reproduce:

  1. Visit about:bookmarks
  2. Have enough folders that it causes scrolling (make window less tall if needed)
  3. Make the screen very wide. Scrollbars will eventually go in wrong place

• Actual result:
  See screenshot

• Expected result:
  Scrollbar should line up w/ edge

• Will the steps above reproduce in a fresh profile? If not what other info can be added?

• Is this an issue in the currently released version?

• Can this issue be consistently reproduced?

• Extra QA steps:
  1.
  2.
  3.

• Screenshot if needed:
  

• Any related issues:

[bsclifton]

@cezaraugusto I believe this is happening after #8532

I assigned 0.15.3 in case you had time. If not, it's all good- this is low priority and you can remove the milestone (this might even make a good first bug). Thanks 😄