[Paint Preview] Add startup experiment to about:flags

This adds the startup experiment flag to about:flags page.

Bug: 1064011
Change-Id: Ib7f01faac121a942a6753f78f37fdb0186825a0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219463
Commit-Queue: Mehran Mahmoudi <[email protected]>
Reviewed-by: Calder Kitagawa <[email protected]>
Cr-Commit-Position: refs/heads/master@{#778450}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index f668b74ad..e3e77878 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -2440,6 +2440,8 @@
 #if BUILDFLAG(ENABLE_PAINT_PREVIEW) && defined(OS_ANDROID)
 extern const char kPaintPreviewDemoName[];
 extern const char kPaintPreviewDemoDescription[];
+extern const char kPaintPreviewStartupName[];
+extern const char kPaintPreviewStartupDescription[];
 #endif  // ENABLE_PAINT_PREVIEW && defined(OS_ANDROID)
 
 #if BUILDFLAG(ENABLE_PLUGINS)