EXO: Modify exo/seat for cast, add shell_surface_util

Prevented ChromeOS-specific funcionality in seat.cc
from compiling for other platforms. To be used for EXO on cast shell.

Added shell_surface_util, containing utility functions previously
included in shell_surface_base, needed for exo on cast.

TBR=yusukes@chromium.org

Bug: 896710
Test: Run exo_unittests and unit_tests on ChromeOS build.
Change-Id: I3d66b09a6e236e731913ecc77cd0fdbe7937e74a
Reviewed-on: https://chromium-review.googlesource.com/c/1327562
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611215}
23 files changed