cast: webview: Update default initialization

* aura::Window default property values return 0 as the default value if
  the property isn't defined. So we can't initialize the IDs to 0 by
  default otherwise we could get matches on un-initialized Webviews.
* Register the WebviewWindowManager::Observer when the Webview is
  created since this is when we have the Webview to parent. If it
  happens that we get a parenting request without a valid Webview it
  will crash the browser process.

Bug: b/132811925
Test: Try to register Webview with ID=0
Change-Id: Ieb82c97be3850633b02894cb2a805272d38d227e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1827582Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700293}

chromecast/browser/webview/webview_grpc_service.cc

@@ -82,8 +82,10 @@ class WebviewRpcInstance : public WebviewController::Client,
 
   WebviewWindowManager* window_manager_;
 
-  int app_id_ = 0;
-  int window_id_ = 0;
+  // Initialize to negative values since the aura::Window properties use 0 as
+  // the default value if the property isn't found.
+  int app_id_ = -1;
+  int window_id_ = -1;
 
   DISALLOW_COPY_AND_ASSIGN(WebviewRpcInstance);
 };
@@ -111,7 +113,6 @@ WebviewRpcInstance::WebviewRpcInstance(
                                           base::Unretained(this));
 
   service_->RequestCreateWebview(&ctx_, &io_, cq_, cq_, &init_callback_);
-  window_manager_->AddObserver(this);
 }
 
 WebviewRpcInstance::~WebviewRpcInstance() {
@@ -181,6 +182,10 @@ bool WebviewRpcInstance::Initialize() {
 void WebviewRpcInstance::CreateWebview(int app_id, int window_id) {
   app_id_ = app_id;
   window_id_ = window_id;
+  // Only start listening for window events after the Webview is created. It
+  // doesn't make sense to listen before since there wouldn't be a Webview to
+  // parent.
+  window_manager_->AddObserver(this);
 
   content::BrowserContext* browser_context =
       shell::CastBrowserProcess::GetInstance()->browser_context();