Files app: Select My files when there are no volumes

Make Files app select "My files" when there are no available volumes,
this to allow Files app to behave properly when volumes subsequently
become available.

Change DirectoryModel.onVolumeInfoListUpdated_ method to check for
non-null |displayRoot| before trying to change to |displayRoot|. This
fixes the error "Cannot read property 'getParent' of null" when Drive
volume becomes available before Downloads volume, which is the default
volume/root.

Change FakeDriveFs to unbind the two mojo bindings if they're bound, so
DriveFsTestVolume can re-mount itself.

Test: browser_tests --gtest_filter="*/fileDisplayWithoutVolumesThenMount*"
Bug: 893161, 884967
Change-Id: Ic813b25261530495c11c9f641a92f6e07f883702
Reviewed-on: https://chromium-review.googlesource.com/c/1272418
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599026}
6 files changed