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/common/command_line_impl.cc

@@ -18,6 +18,11 @@ CefCommandLineImpl::CefCommandLineImpl(base::CommandLine* value,
           read_only,
           nullptr) {}
 
+CefCommandLineImpl::CefCommandLineImpl(const base::CommandLine& value)
+    : CefCommandLineImpl(const_cast<base::CommandLine*>(&value),
+                         /*will_delete=*/false,
+                         /*read_only=*/true) {}
+
 bool CefCommandLineImpl::IsValid() {
   return !detached();
 }