Folders and Titles lists in about:bookmarks should scroll independently

[CliqueBait]

Test Plan:

#8532 (comment)

• Have a lot of bookmarks
• Go to about:bookmarks
• Ensure that:
  • Layout is fixed
  • You can scroll between folders (left-hand side column) and titles (right-hand side column) wouldn't scroll together
  • Less-than-screen-height number of bookmakrs shouldn't let you scroll (layout is fixed)
  • At small sizes we fallback to what we have and you can scroll gracefully

Original issue description

I have several dozen bookmark folders and have several dozen bookmarks in each folder. Re-organising bookmarks is impractical and clumsy without being able to scroll the two separately as is the case in Chromium (and other browsers).

Ubuntu 16.10
Brave: 0.12.5
Electron: 1.4.14
libchromiumcontent: 53.0.2785.143
V8: 5.3.332.47
Node.js: 6.5.0
Update Channel: dev

[CliqueBait]

Issue raised again by a user in the brave community:
https://community.brave.com/t/issue-with-bookmark-manager/530

[bsclifton]

This would be a great one to have! I personally have a large number of bookmarks and trying to organize them is painful (because of the constant need to scroll up and down)

[luixxiul]

I thought this was duping to #2076. It must have been removed at some point while the bookmark manager was updated :-)

[cndouglas]

Transferred milestone from duplicate #8441.