[Chromoting] Add Mac host multimon support.

There are 5 parts to this cl:
(1) Give DesktopCapturerProxy a ref to the ClientSessionControl so that
it can send display info to the ClientSession
(2) Query Mac OS for display info
(3) Fixup Mac display orientation (move origin from bottom-left to
top-left)
(4) Add display id to display info. On Windows, the display id is the
index into the array, but Mac has a separate identifier we need to track.
(5) Restrict the desktop to only those displays that match the main
display's DPI. This is to match webrtc's behavior.

Change-Id: Ia71c537bf521c1ccfcb1e4949792a61433160d3f
Reviewed-on: https://chromium-review.googlesource.com/c/1479281
Reviewed-by: Joe Downing <[email protected]>
Commit-Queue: Gary Kacmarcik <[email protected]>
Cr-Commit-Position: refs/heads/master@{#634777}
diff --git a/remoting/proto/control.proto b/remoting/proto/control.proto
index c020e6a..8829120 100644
--- a/remoting/proto/control.proto
+++ b/remoting/proto/control.proto
@@ -85,6 +85,9 @@
 }
 
 message VideoTrackLayout {
+  // Unique display identifier.
+  optional int32 id = 8;
+
   // Name of the video track.
   optional string track_name = 1;