DevTools: Add context menu entries for Request Payload toggles
DevTools: Add context menu entries for Request Payload toggles

In the the Network Headers pane, the buttons for "View source/parsed"
were not accessible via keyboard. This change adds context menu entries
to the Request Payload list item, so a keyboard user can access these
toggles using a context menu key or shortcut.

Note: this does not fix the toggles for other sections in the headers
pane

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/C350Blj
Bug: 963183
Change-Id: I3c5807b66d65da2b1f0f1dbfa84e761036104851
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1880518
Commit-Queue: Amanda Baker <fef0829ad7bffb00281b036cb285679c0544f52e@microsoft.com>
Reviewed-by: Lorne Mitchell <b024a4e2ee56b9b5450225fa7affa8f95408a018@microsoft.com>

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