Disable metalayer tool when assistant is not available

BUG=b:68664196
TEST=unit tests and manually check on device

Change-Id: If4c997a048ffc7ec3afa3e9ab2199f8c2b140c7a
Reviewed-on: https://chromium-review.googlesource.com/828265
Reviewed-by: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524881}
const gfx::VectorIcon& MetalayerMode::GetPaletteIcon() const {
  return kPaletteModeMetalayerIcon;
}

const gfx::VectorIcon& MetalayerMode::GetPaletteIcon() const {
  return kPaletteModeMetalayerIcon;
}
