Add .md for Windows Shortcut and taskbar handling

Change-Id: I18d23eb64d576ea1e9ff443f2de7b9c0c03da81f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2448664
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#813866}

docs/windows_shortcut_and_taskbar_handling.md

+# Windows Shortcut and Pinned Taskbar Icon handling
+
+When Chrome is installed on Windows, it creates a shortcut on the desktop that
+launches Chrome. It also adds the same shortcut to the start menu. These
+shortcuts do not specify a profile, so they launch Chrome with the most recently
+used profile. 
+
+Windows allows users to pin applications to the taskbar. When a user
+pins an application to the taskbar, Windows looks for a desktop shortcut that
+matches the application, and if it finds one, it creates a .lnk file in the
+directory
+`<user dir>\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar.`
+If it does not find a matching desktop shortcut, it creates an 8-hex-digit
+sub-directory of
+`<user dir>\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\ImplicitAppShortcuts\`
+and puts the .lnk file in that directory. For example, 3ffff1b1b170b31e.
+
+App windows on Windows have an
+[App User Model ID (AUMI)](https://docs.microsoft.com/en-us/windows/win32/shell/appids)
+property. For Chrome windows, this is set in
+[BrowserWindowPropertyManager::UpdateWindowProperties](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/views/frame/browser_window_property_manager_win.cc?q=BrowserWindowPropertyManager::UpdateWindowProperties),
+when a window is opened. Windows desktop shortcuts have an app model property,
+and this should match the open window's AUMI. Windows groups open windows with
+the same AUMI to a taskbar icon.
+
+There are two kinds of Chrome windows with AUMI's: browser windows, and app
+windows, which include web apps, and extensions, i.e., windows opened via
+--app-id or --app.
+
+[GetAppUserModelIdForBrowser](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/shell_integration_win.cc?q=GetAppUserModelIdForBrowser)
+constructs an AUMI for a browser window and
+[GetAppUserModelIdForApp](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/shell_integration_win.cc?q=GetAppUserModelIdForApp)
+constructs an AUMI for an app window. Each calls
+[ShellUtil::BuildAppUserModelId](https://source.chromium.org/chromium/chromium/src/+/master:chrome/installer/util/shell_util.cc;q=ShellUtil::BuildAppUserModelId)
+to construct the AUMI out of component strings.
+
+All AUMI's start with the base app id,
+[install_static::GetBaseAppId](https://source.chromium.org/chromium/chromium/src/+/master:chrome/install_static/install_util.cc?q=install_static::GetBaseAppId).
+This varies for different Chrome channels (e.g., Canary vs. Stable) and
+different Chromium-based browsers (e.g., Chrome vs. Chromium).
+
+The AUMI for a browser app has the format:
+`<BaseAppId>.<app_name>[.<profile_name>]`.
+profile_name is only appended when it's not the default profile.
+
+The AUMI for a Chrome browser window has the format:
+`<BaseAppId>[browser_suffix][.profile_name]`.
+profile_name is only appended when it's not the default profile.
+browser_suffix is only appended to the BaseAppId if the installer
+has set the kRegisterChromeBrowserSuffix command line switch, e.g.,
+on user-level installs.
+
+Since AUMI's for browser and app windows include the profile_name, each
+profile's windows will be grouped together on the taskbar.
+
+shell_integration_win.cc has a function [GetExpectedAppId](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/shell_integration_win.cc?q=GetExpectedAppid)
+to determine what the AUMI for a shortcut should be. It also has a function
+[MigrateTaskbarPins](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/shell_integration_win.cc?q=MigrateTaskbarPins)
+to migrate pinned taskbar icons if the AUMI's need to change.
+
+## Multi-profile Support
+When the user has more than one profile, the shortcuts are renamed to include
+the profile name, e.g., `Chrome.lnk` becomes `<profile name> - Chrome`.  The
+shortcut icons, both desktop and taskbar, are badged with their profile icon.
+This badged icon is also used in the tab preview for a Chrome window.
+
+## Diagnosing Issues
+To dump a taskbar icon's properties, run this command:
+
+`python \src\chromium\src\chrome\installer\tools\shortcut_properties.py --dump-all <user dir>\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar`
+
+This shows you the properties of all the taskbar pinned icons. If the taskbar
+icon is in a subdirectory of ImplicitApps, pass that directory to
+shortcut_properties.py.
+