Improve SystemWebDialogDelegate documentation

Add class level comments in |SystemWebDialogDelegate| and
|InlineLoginHandlerDialogChromeOS| to explain why they are intentionally
made movable.

Additionally fix an include lint error in
inline_login_handler_dialog_chromeos.h.

Bug: 833797
Change-Id: I428472f885b204ef73aab06853614febbc1ccadd
Reviewed-on: https://chromium-review.googlesource.com/c/1296608
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602179}

chrome/browser/ui/webui/chromeos/system_web_dialog_delegate.h

@@ -6,14 +6,16 @@
 #define CHROME_BROWSER_UI_WEBUI_CHROMEOS_SYSTEM_WEB_DIALOG_DELEGATE_H_
 
 #include <string>
+#include <vector>
 
 #include "base/macros.h"
 #include "base/strings/string16.h"
 #include "ui/web_dialogs/web_dialog_delegate.h"
 #include "url/gurl.h"
 
-// ui::WebDialogDelegate for system Web UI dialogs, e.g. dialogs opened from
-// the ash system tray.
+// ui::WebDialogDelegate for always-on-top system Web UI dialogs, e.g. dialogs
+// opened from the ash system tray. These dialogs are intentionally movable /
+// draggable so that content from other pages can be copy-pasted.
 
 namespace chromeos {
 

chrome/browser/ui/webui/signin/inline_login_handler_dialog_chromeos.h

@@ -12,6 +12,9 @@
 
 namespace chromeos {
 
+// Extends from |SystemWebDialogDelegate| to create an always-on-top but movable
+// dialog. It is intentionally made movable so that users can copy-paste account
+// passwords from password managers.
 class InlineLoginHandlerDialogChromeOS : public SystemWebDialogDelegate {
  public:
   static void Show();