Add a dismissModals method to ContentSuggestions

This CL implements the dismissModals method needed by the
NewTabPagePanelProtocol, and merges the existing ones with it.

BUG=700375

Change-Id: I95ff37f649fb6a1e6ab29d3f3ed917d74883c80d
Reviewed-on: https://chromium-review.googlesource.com/558873
Reviewed-by: Louis Romero <lpromero@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483981}