EXO: Refactor wp_viewporter interface

Move the wp_viewporter interface into its own files. This
change only moves code around, no functional changes.

Bug: 896710
Test: Build and run exo_unittests
Change-Id: Ie9e2ea1a52ab26f77da3eca392f96d9099198fbc
Reviewed-on: https://chromium-review.googlesource.com/c/1481061
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634792}
4 files changed