[Chromoting] Fix mouse when single display selected in multi-mon

Previous code would always translate the mouse coords relative to the
upper-left corner of the entire display, instead of relative to the
currently selected display.

This cl changes the mouse input scaler to be a DesktopRect instead of
a DesktopSize because we need to keep track of the origin for proper
mouse event scaling.

Change-Id: I397c51280187c7a8e023e8a41940b6f37b39eb7f
Reviewed-on: https://chromium-review.googlesource.com/c/1429339
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625325}
11 files changed