chromeos: Make multi-profile add user dialog work with mash

For go/mustash we have to use mojo interfaces to communicate between
the ash process and chrome browser. Add a mojo method to show the
dialog.

Migrate code from SystemTrayDelegateChromeOS (deprecated and doesn't
exist in mash) and ChromeShellClient (also doesn't exist in mash)
into SessionControllerClient.

This helps consolidate more session-related code into
SessionControllerClient and reduce the size of SystemTrayDelegate.

[email protected]

Bug: 647412
Test: existing ash and chrome tests
Change-Id: I1f65f43a4fe668095325ad20c8dc0c201f706321
Reviewed-on: https://chromium-review.googlesource.com/557935
Reviewed-by: James Cook <[email protected]>
Reviewed-by: Tom Sepez <[email protected]>
Reviewed-by: Michael Wasserman <[email protected]>
Commit-Queue: James Cook <[email protected]>
Cr-Commit-Position: refs/heads/master@{#485334}
diff --git a/ash/mus/shell_delegate_mus.cc b/ash/mus/shell_delegate_mus.cc
index 44eefd4d..f99e7c4 100644
--- a/ash/mus/shell_delegate_mus.cc
+++ b/ash/mus/shell_delegate_mus.cc
@@ -40,7 +40,7 @@
 
 bool ShellDelegateMus::IsMultiProfilesEnabled() const {
   NOTIMPLEMENTED();
-  return false;
+  return true;  // For manual testing of multi-profile under mash.
 }
 
 bool ShellDelegateMus::IsRunningInForcedAppMode() const {