Make first item in sources panel placeholder not keyboard navigatable
Make first item in sources panel placeholder not keyboard navigatable

None of the other items except for the workspaces help text is keyboard
navigatable, and exist as hints for keyboard shortcuts. This change
makes the behavior consistent.

R=024a1e0af501338d11c3b837c6193d11c140d792@chromium.org

Fixed: 1192484
Change-Id: I6543e40906f8c5b71db35ef9b09e76a3f6966372
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2813080
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Reviewed-by: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>
Commit-Queue: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend