[System PWAs] Add about:flags entry.

This CL adds an about:flags entry for the SystemWebApps feature.

Bug: 836128
Change-Id: I83ae0dcd55885ca5e95fe10fb6384aa844ae25df
Reviewed-on: https://chromium-review.googlesource.com/1240055
Reviewed-by: Alan Cutter <[email protected]>
Commit-Queue: calamity <[email protected]>
Cr-Commit-Position: refs/heads/master@{#594181}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 01eec93..e0453ed 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -339,6 +339,9 @@
 extern const char kEnableDesktopPWAsLinkCapturingName[];
 extern const char kEnableDesktopPWAsLinkCapturingDescription[];
 
+extern const char kEnableSystemWebAppsName[];
+extern const char kEnableSystemWebAppsDescription[];
+
 extern const char kEnableDockedMagnifierName[];
 extern const char kEnableDockedMagnifierDescription[];