WebDriver: Wait focus to complete when switching window (#38160)

Previously the webdriver do not wait for focus to complete, which can
cause some instability.

No matter interact as human or webdriver, the focus chain always goes
as: Embedder forwards -> Constellation (do some updates) -> Embedder (do
some updates).

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

components/shared/constellation/lib.rs

@@ -68,8 +68,9 @@ pub enum EmbedderToConstellationMessage {
     CloseWebView(WebViewId),
     /// Panic a top level browsing context.
     SendError(Option<WebViewId>, String),
-    /// Make a webview focused.
-    FocusWebView(WebViewId),
+    /// Make a webview focused. If sender is provided, it will be used to send back a
+    /// bool indicating whether the focus was successfully set in EmbedderMsg::WebViewFocused.
+    FocusWebView(WebViewId, Option<IpcSender<bool>>),
     /// Make none of the webviews focused.
     BlurWebView,
     /// Forward an input event to an appropriate ScriptTask.

components/shared/embedder/lib.rs

@@ -373,7 +373,9 @@ pub enum EmbedderMsg {
     /// A webview was destroyed.
     WebViewClosed(WebViewId),
     /// A webview gained focus for keyboard events
-    WebViewFocused(WebViewId),
+    /// If sender is provided, it will be used to send back a
+    /// bool indicating whether the focus was successfully set.
+    WebViewFocused(WebViewId, Option<IpcSender<bool>>),
     /// All webviews lost focus for keyboard events.
     WebViewBlurred,
     /// Wether or not to unload a document

components/shared/embedder/webdriver.rs

@@ -152,7 +152,8 @@ pub enum WebDriverCommandMsg {
     /// Close the webview associated with the provided id.
     CloseWebView(WebViewId),
     /// Focus the webview associated with the provided id.
-    FocusWebView(WebViewId),
+    /// Sends back a bool indicating whether the focus was successfully set.
+    FocusWebView(WebViewId, IpcSender<bool>),
     /// Get focused webview.
     GetFocusedWebView(IpcSender<Option<WebViewId>>),
     /// Check whether top-level browsing context is open.