[Chromoting] Cache display DPI

This contains a workaround for the DPI==0 problem on some user's
machines. Rather than assume that the dpi=96, this caches the
DPI value from the OnVideoSizeChanged message and uses that as
the default when the display DPI==0.

Also, add additional display DPI logging for debugging.

Change-Id: I6afb7c1ffbfe3a4ad1881a7702ff427cde460c9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612031
Auto-Submit: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659988}
6 files changed