Add FakeDisplaySnapshot builder and related changes.

Adds a builder for FakeDisplaySnapshot to allow us to easily construct
the display snapshots. Will update more tests to use this functionality
in a follow up CL.

Update platform_screen_ozone_unittests.cc to use FakeDisplaySnapshot
instead of DisplaySnapshotVirtual.

Update --screen-config to take display DPI and able to start without any
initial displays.

BUG=611475

Review-Url: https://codereview.chromium.org/2340383002
Cr-Commit-Position: refs/heads/master@{#419885}
6 files changed