Perf dashboard should auto-generate manifest file when one is missing

Perf dashboard should auto-generate manifest file when one is missing
https://bugs.webkit.org/show_bug.cgi?id=152813

Reviewed by Chris Dumez.

When /data/manifest.json is missing, fall back to newly added /api/manifest instead of
silently failing to show the UI. This will make the initial setup easier.

* public/api/manifest.php: Added.
(main):
* public/include/manifest.php:
(Manifest::manifest): Added.
* public/v3/main.js:
(fetchManifest):
(didFetchManifest): Extracted from fetchManifest.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@194679 268f45cc-cd09-0410-ab3c-d52691b4dbfc