Adding a confirmation dialog for It2Me on Linux

This change adds a confirmation dialog for the It2Me Linux host.  This
dialog will be displayed when an incoming It2Me connection and will wait
up to 60 seconds for the user to accept/reject it.  If the user accepts
the connection then we will complete the conneciton process, otherwise
the incoming request will be rejected.

BUG=645540

Review-Url: https://codereview.chromium.org/2343443002
Cr-Original-Commit-Position: refs/heads/master@{#418611}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6243fba501529b041abd75c9554157d70bd14d74

config/linux/gtk2/BUILD.gn

@@ -28,6 +28,7 @@ group("gtk2") {
     "//chrome/browser/ui/libgtk2ui",
     "//gpu/gles2_conform_support:gles2_conform_test_windowless",
     "//remoting/host",
+    "//remoting/host/it2me:common",
     "//remoting/host/it2me:remote_assistance_host",
     "//remoting/host:remoting_me2me_host_static",
     "//remoting/test:it2me_standalone_host_main",