DevTools: Add keyboard navigation in Request Payload object
DevTools: Add keyboard navigation in Request Payload object

Previously, in the Request Payload section of the Headers tab in the
Network tool, an ObjectPropertiesSection was inserted as a tree within a
tree element, which prevented keyboaring in the object itself. This
means that keyboard users were blocked from being able to access the
content of the object since only the root node is expanded by default.

This change appends the root node of the object into the existing tree,
so users can navigate through the elements of the object just the same
as the rest of the items in the tree of the Headers tab.

Testing: Go to youtube, open Network tool and refresh the page. There
should be an entry with the name "log_event?alt=json&key=..." which has
a request payload for verifying this change.

Screenshot: https://imgur.com/a/s5wwb7t

Bug: 963183
Change-Id: I7df639f97d4f807e25af854fc8a16f097837a1e0
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1880516
Commit-Queue: Amanda Baker <fef0829ad7bffb00281b036cb285679c0544f52e@microsoft.com>
Reviewed-by: Lorne Mitchell <b024a4e2ee56b9b5450225fa7affa8f95408a018@microsoft.com>

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