Use side-panel's bookmark part as sidebar's bookmark panel

[simonhong]

Load below side panel's bookmark part to sidebar bookmarks

[simonhong]

side-panel page is loaded in sidebar like below.
However, that page doesn't work properly.
Ex) Add current tab's state is not updated. Context menu is not rendered.

[simonhong]

Found the reason why some features of read-later page(brave://read-later.top-chrome) doesn't work in the sidebar panel
after loading its url in the sidebar panel's webcontents.
chromium uses customized webview for sidepanel(ReadLaterSidePanelWebView) that monitors active tab changing,
providing custom context menu and set BubbleContentsWrapper::Host toReadLaterUI.
However, sidebar uses one WebView instance with different WebContents for each builtin panel.
To make sidepanel's read-later page works at sidebar, sidebar bookmarks panel should provide similar logic what ReadLaterSidePanelWebView does.

[simonhong]

One more thing - we may want to display side panel's bookmark part at sidebar regardless of upstream's side-panel feature flag state.

[stephendonner]

Verified PASSED using

Brave	1.37.36 Chromium: 98.0.4758.87 (Official Build) nightly (64-bit)
Revision	e4cd00f135fb4d8edc64c8aa6ecbe7cc79ebb3b2-refs/branch-heads/4758@{#1002}
OS	Windows 10 Version 21H2 (Build 19044.1466)

Steps:

1. new profile
2. open brave://flags/
3. set Enable Sidebar to Enabled
4. restart Brave
5. click on the Bookmarks icon in Sidebar

Tested the following:

Context-menu

• Open in New Tab
• Open in New Window
• Open in Private Window

---

• Edit...

---

• Cut
• Copy
• Paste

---

• Delete

---

• Add Page...
• Add Folder...

---

• Bookmark Manager

drag and drop from brave://bookmarks <-> Bookmarks in Sidebar
drag and drop from desktop -> Bookmarks in Sidebar

example	example	example	example	example

example	example	example	example	example

Also tested multi-selection context menu items (Open all (9))

Filed:
#20995
#20996
#21000
#21004

And still testing 😉

---

Verification passed on

Brave	1.37.86 Chromium: 99.0.4844.51 (Official Build) beta (64-bit)
Revision	d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS	Ubuntu 18.04 LTS

Verified the following:

• Open in New Tab
• Open in New Window
• Open in Private Window
• Edit...
• Cut
• Copy
• Paste
• Delete
• Add Page...
• Add Folder...

Example:

Drag and drop within bookmark panel

Drag and drop between bookmark panel and bookmark manager

---

Verified PASSED using the above testcases with

Brave	1.37.91 Chromium: 99.0.4844.51 (Official Build) beta (x86_64)
Revision	d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS	macOS Version 12.3 (Build 21E230)