EXO: Move CaptureClient getter in WMHelper

ChromeOS and Chromecast use different capture clients. This allows
platforms to inject their own capture clients via the WMHelper
abstraction.

Bug: 896710
Test: Run exo_unittests
Change-Id: Ic7dd5737529db53e35ef6f5192fc86af04bb28c7
Reviewed-on: https://chromium-review.googlesource.com/c/1362246
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614138}
10 files changed