[Chromoting] Set the window size to use only the available screen area.

[Chromoting] Set the window size to use only the available screen area.

Most platforms reserve some of the screen area for use by the OS (on Mac
and some Linux WM's there is a menu bar; on all platforms there is some
sort of application launcher). Since the window must be positioned outside
this reserved area, creating a window as large as the entire screen causes
it to extend beyond the bounds of the screen (or monitor), which results
in some client-side UI attached to the edge of the window being off-screen.

NOTRY=True
BUG=

Review URL: https://codereview.chromium.org/841603003

Cr-Commit-Position: refs/heads/master@{#310216}