GitLab

Use base::Optional for startup and application ID in order to
handle null values properly. Setting one of these to null will
now clear the window property.

Bug: 834971
Test: exo_unittests --gtest_filter=ShellSurfaceTest.Set*Id
Change-Id: I395fb02200f310b027060eb28e0132d8976532ab
Reviewed-on: https://chromium-review.googlesource.com/1020042Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552178}