Introduce the ability to wait for the watcher process to initialize.

components/browser_watcher/watcher_client_win.cc

Index: components/browser_watcher/watcher_client_win.cc
diff --git a/components/browser_watcher/watcher_client_win.cc b/components/browser_watcher/watcher_client_win.cc
index e055278839034731404114449c422e0ef06efe98..dd6182a391bac6a1778f223d0de90107ef58eaa3 100644
--- a/components/browser_watcher/watcher_client_win.cc
+++ b/components/browser_watcher/watcher_client_win.cc
@@ -48,6 +48,8 @@ void WatcherClient::LaunchWatcher() {
     // Launch the child process inheriting only |self| on
     // Vista and better.
     to_inherit.push_back(self.Handle());
+    to_inherit.insert(to_inherit.end(), inherited_handles_.begin(),
+                      inherited_handles_.end());
     options.handles_to_inherit = &to_inherit;
   }
 
@@ -56,4 +58,8 @@ void WatcherClient::LaunchWatcher() {
     LOG(ERROR) << "Failed to launch browser watcher.";
 }
 
+void WatcherClient::AddInheritedHandle(HANDLE handle) {
+  inherited_handles_.push_back(handle);
+}
+
 }  // namespace browser_watcher