DevTools: Allow multiple ObjectPropertiesSections in one tree
DevTools: Allow multiple ObjectPropertiesSections in one tree

In response to feedback on [1], this CL refactors the Watch expressions
and Scope pane to keep all of their respective objects in one tree each
instead of one tree per object. This allows simple keyboard navigation
between watch expressions or scopes and creates more clarity for screen
reader users. This change also allows users to edit watch expressions
and editable object properties by pressing the enter key in order to
make that functionality keyboard-accessible.

There is still an issue where data-keyboard-focus is not applied when
the tree element is selected after exiting a text prompt or context
menu, but I believe that is an unrelated issue with treeoutline or
UIUtils that can be fixed separately.

Before: https://gyazo.com/656714c5e92b268e8a8a988d155bd592
After: https://gyazo.com/02e0df16e66c188c3c8f8d5b38dd3b42

Navigation gif: https://i.imgur.com/I5qT0UW.mp4

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/1646644/9/third_party/blink/renderer/devtools/front_end/sources/WatchExpressionsSidebarPane.js#463

Bug: 963183
Change-Id: Iedb6d941364c457cef7614dae6a75ca6b8165c56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719852
Reviewed-by: Lorne Mitchell <b024a4e2ee56b9b5450225fa7affa8f95408a018@microsoft.com>
Commit-Queue: Jack Lynch <d6fb7cfbb8c8618e756350219cb7952a0ad656b7@microsoft.com>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#704709}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0f0a0c865b3720d3c59a874ec90dd40617d141ef

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