This tool shows/hides the "metalayer" overlay on the
containter side.

The tool name string is temporary, as well as the
palette icons (reusing the icons from "Capture region").

Still keeping around the temporary Shift-Search-A
shortcut and the repurposed "Capture region" tool.

BUG=b:37165189
TEST="Metalayer mode" stylus tool toggles the metalayer.

Review-Url: https://codereview.chromium.org/2824703006
Cr-Commit-Position: refs/heads/master@{#466167}