Restart crashpad_handler on Mac if it exits unexpectedly

This feature is only operative on OS X 10.10 and later, as directed by
CrashpadClient::StartHandler(). This limitation is enforced because the
handler restarter can cause the browser process to become an unreapable
pseudo-zombie on earlier OS versions if it crashes while a handler
restart is pending.

Review URL: https://codereview.chromium.org/1470073002

Cr-Commit-Position: refs/heads/master@{#361201}

components/crash/content/app/crashpad_mac.mm

@@ -153,7 +153,7 @@ void InitializeCrashpad(bool initial_client, const std::string& process_type) {
                                                  url,
                                                  process_annotations,
                                                  arguments,
-                                                 false);
+                                                 true);
       if (result) {
         result = crashpad_client.UseHandler();
       }