Create SingleAppHost.md

Author: LiangTheDev

specs/SingleAppHost.md

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+# Background
+While WebViews from different app processes normally don't share the same WebView browser process instance, app processes from the same product suite may share the same webview browser process
+instance by specifying the same user data folder when creating WebView2Environment object.
+The `IsForSingleAppHost` property is for the developer to express the sharing intent, so that we could provide optimized security and performance according expected usage.
+
+# Description
+The `IsForSingleAppHost` property indicates whether other processes can create WebView2 sharing the same WebView browser process instance by using WebView2Environment created with the same user data folder.
+Default is FALSE, meaning that other processes can create WebView objects sharing the same browser process instance.
+
+# Examples
+## Win32 C++
+```cpp
+    auto options = Microsoft::WRL::Make<CoreWebView2EnvironmentOptions>();
+    CHECK_FAILURE(options->put_IsForSingleAppHost(TRUE);
+    HRESULT hr = CreateCoreWebView2EnvironmentWithOptions(
+        nullptr, m_userDataFolder.c_str(), options.Get(),
+        Callback<ICoreWebView2CreateCoreWebView2EnvironmentCompletedHandler>(
+            this, &AppWindow::OnCreateEnvironmentCompleted).Get());
+```
+## WinRT and .NET
+```c#
+auto options = new CoreWebView2EnvironmentOptions();
+options.IsForSingleAppHost = true;
+auto environment = await CoreWebView2Environment.CreateAsync(BrowserExecutableFolder, UserDataFolder, options);
+```
+
+# Remarks
+See API details.
+
+# API Details
+## Win32 C++
+```cpp
+interface ICoreWebView2EnvironmentOptions_2 : IUnknown {
+
+  /// Whether other processes can create WebView2 sharing the same WebView browser
+  /// process instance by using WebView2Environment created with the same user data folder.
+  /// Default is FALSE, meaning that other processes can create WebView objects
+  /// sharing the same browser process instance.
+  [propget] HRESULT IsForSingleAppHost([out, retval] BOOL* value);
+
+  /// Sets the `ForSingleAppHost` property.
+  /// When set as TRUE, no other process can create WebView sharing the same browser
+  /// process instance. When another process tries to create WebView2Controller from
+  /// an WebView2Environment objct created with the same user data folder, it will fail
+  /// with `HRESULT_FROM_WIN32(ERROR_INVALID_STATE)`.
+  /// When set to TRUE, `TrySuspend` could potentially do more optimization on reducing
+  /// CPU usage for suspended WebViews.
+  [propput] HRESULT IsForSingleAppHost([in] BOOL value);
+
+}
+
+```
+## WinRT and .NET
+```c#
+unsealed runtimeclass CoreWebView2EnvironmentOptions
+{
+    // ..
+    bool IsForSingleAppHost { get; set; };
+}
+```