chrome: Add callback for already running app relaunch (fixes #3609)

Adds a new CefBrowserProcessHandler::OnAlreadyRunningAppRelaunch
callback for when an already running app is relaunched with the
same CefSettings.root_cache_path.

Client apps should check the CefInitialize() return value for early
exit of the relaunch source process.

libcef/browser/main_runner.h

@@ -59,7 +59,7 @@ class CefMainRunner : public CefMainRunnerHandler {
   int ContentMainInitialize(const CefMainArgs& args,
                             void* windows_sandbox_info,
                             int* no_sandbox);
-  bool ContentMainRun(bool* initialized, base::OnceClosure context_initialized);
+  int ContentMainRun(bool* initialized, base::OnceClosure context_initialized);
 
   // CefMainRunnerHandler methods:
   void PreBrowserMain() override;