Add support for Icon in  ToolbarToggle
Add support for Icon in  ToolbarToggle

This CL adds support for icons in ToolbarToggle by following the
same approach as we use in ToolbarButton.

Change-Id: I0d2af45921a9a4ab96bc5e0e17e084c1987178b6
Bug: none
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3484210
Reviewed-by: Alex Rudenko <d8d88e7d7e77f69e23d1d1432231196fb8e02a62@chromium.org>
Commit-Queue: Johan Bay <c4a91c97e1f8a218202ca733f9e9f5122f475e0f@chromium.org>

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