Change exo/WMHelper into an interface

Allows OS-specific implementations of WMHelper
Change ChromeOS-specific files to use WMHelperChromeOS.
This CL does not change functionality.

Bug: 896710
Test: Built and ran on ChromeOS.
Change-Id: I3e2970313e85b07fdab97bd6e8645d8d3b23b40d
Reviewed-on: https://chromium-review.googlesource.com/c/1292602
Commit-Queue: Shawn Gallea <sagallea@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605398}
14 files changed