Make SelectionPopupController.ShowPastePopup only be triggered by Blink

This patch is progress towards the goal of having Blink triggering all
of clank's SelectActionMode context menus. In particular this patch
has Blink trigger the menu when an insertion handle is tapped or
dragged.

BUG=627234

Review-Url: https://codereview.chromium.org/2721813002
Cr-Commit-Position: refs/heads/master@{#454203}
7 files changed