[OverlayWindow] Add platform-independent window and views implementation.

chrome/browser/ui/BUILD.gn

Index: chrome/browser/ui/BUILD.gn
diff --git a/chrome/browser/ui/BUILD.gn b/chrome/browser/ui/BUILD.gn
index f2ce0a8afd5fa2364c095f2b1c9393b3a45aa157..806ca4844bcce668b93d4fe991da44e603d87c4c 100644
--- a/chrome/browser/ui/BUILD.gn
+++ b/chrome/browser/ui/BUILD.gn
@@ -816,6 +816,7 @@ split_static_library("ui") {
       "pdf/chrome_pdf_web_contents_helper_client.h",
       "permission_bubble/chooser_bubble_delegate.cc",
       "permission_bubble/chooser_bubble_delegate.h",
+      "pip/pip_window.h",

[mlamouri (slow - plz ping), 2017/06/05 13:09:33]

Same as below, what about "picture_in_picture/picture_in_picture_window.h"?

FWIW, I don't expect us to have much stuff in that directory as
chrome/browser/ui/ isn't used by Android so it will only implement the desktop
window. The cross-platform implementation will have to be at a different layer.

[apacible, 2017/06/13 07:58:08]

From a previous conversation with pkasting, spelling out picture_in_picture was
considered too wordy.

       "sad_tab.cc",
       "sad_tab.h",
       "sad_tab_helper.cc",
@@ -1596,6 +1597,8 @@ split_static_library("ui") {
       "views/permission_bubble/chooser_bubble_ui.h",
       "views/permission_bubble/permission_prompt_impl.cc",
       "views/permission_bubble/permission_prompt_impl.h",
+      "views/pip/pip_window_views.cc",
+      "views/pip/pip_window_views.h",
       "views/subtle_notification_view.cc",
       "views/subtle_notification_view.h",
       "views/sync/bubble_sync_promo_view.cc",