GitLab

This prototypes the idea of uneliding when the user affirmatively
places the caret within the existing URL or makes a partial selection.

The partial selection can be made either via double-click or by
dragging. The unelision happens on mouseup.

It feels pretty good.

Change-Id: I5522a7b6dc8d94fe84c39c8b71995abc5007651a
Reviewed-on: https://chromium-review.googlesource.com/910110Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537813}