Enable sticky pinned tabs by default to improve vertical scrolling

[sangwoo108]

The default value was disabled. Now we think it's good to go.

---

Verification steps

• Go to brave://flags/#brave-vertical-tabs-stick-pinned-tabs and set it "Default"
• Feature described from Freeze the pane/separator for pinned tab section above vertical tab view panel #29935 should work, and other vertical tabs features should work as expected.

[MadhaviSeelam]

Verification Passed using

Brave | 1.54.75 Chromium: 115.0.5790.32 (Official Build) beta (64-bit)
-- | --
Revision | f9b78cf410ecd5b372310ef25a12ef456c77d90f-refs/branch-heads/5790@{#724}
OS | Windows 11 Version 22H2 (Build 22621.1848)

1. Install 1.54.75
2. launch Brave
3. clicked Set Brave as default Browser
4. imported Profiles from Edge
5. opened brave://flags
6. verified #brave-vertical-tabs-stick-pinned-tabs is set to Default
7. opened few tabs
8. clicked to open https://www.cnbc.com bookmark
9. right clicked on the tab to open context menu
10. selected Use Vertical tabs
11. verified vertical tabs panel is shown with all the open tabs in expanded mode
12. selected Pin from the context menu for one of the tabs (i.e. cnbc.com)
13. pin few more tabs
14. scrolled thru unpinned tabs

• confirmed cnbc.com tab is pinned
• confirmed additional tabs pinned as expected
• confirmed pinned tabs were sticky when scrolling thru unpinned tabs in expanded mode
• confirmed pinned tabs were sticky when scrolling thru unpinned tabs in Float/mouseover mode

example	example	example	example

2023-06-21_18h11_50.mp4