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.

tests/ceftests/frame_unittest.cc

@@ -31,9 +31,6 @@ namespace {
 // 4. FrameNavTestHandler retrieves the URL to load via
 //    FrameNavExpectationsBrowser::GetMainURL and calls either CreateBrowser
 //    (for the first navigation) or LoadURL (for the following navigations).
-// 5. If the renderer process does not already exist CEF creates it with
-//    command-line arguments that specify the FrameNavFactoryId via
-//    FrameNavBrowserTest::OnBeforeChildProcessLaunch.
 //
 // In the renderer process:
 // 6. If the renderer process is newly created FrameNavRendererTest calls