workspace: Exclude non-resizeable windows from the multi-resize feature.

Before this change, a supposedly non-resizeable window could participate
in a multi-resize. Non-resizeable should mean the user has no way to
resize the window.

Test: ash_unittests MultiWindowResizeControllerTest.*NonResizeable*
Bug: 921810
Change-Id: I1e3a24b1267b6afd6b68d666e6ab033ddbd7543c
Reviewed-on: https://chromium-review.googlesource.com/c/1423746
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625246}
3 files changed