[HaTS] Add a feature flag to directly trigger UI

Add a new feature flag HappinessTrackingSurveysForDesktopDemo to
allow designer/dev/testing team to directly invoke the survey
related UI for tests. This flag allows users to bypass the
triggering mechanisms to directly invoke Survey bubble UI on NTP.

BUG=979530
TEST=either use command line
"--enable-features=HappinessTrackingSurveysForDesktopDemo" or by
changing the corresponding flag in chrome://flags, user can directly
see user happiness survey bubble on a New Tab Page.

Change-Id: Ibc17ba729fb01cb33c9245ba25ce1dc2cbbf304c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1759287
Reviewed-by: Peter Kasting <[email protected]>
Reviewed-by: Robert Liao <[email protected]>
Commit-Queue: Wei Li <[email protected]>
Cr-Commit-Position: refs/heads/master@{#688280}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index e94d38f..333b299 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -2780,6 +2780,11 @@
 const char kHappinessTrackingSurveysForDesktopDescription[] =
     "Enable showing Happiness Tracking Surveys to users on Desktop";
 
+const char kHappinessTrackingSurveysForDesktopDemoName[] =
+    "Happiness Tracking Surveys Demo";
+const char kHappinessTrackingSurveysForDesktopDemoDescription[] =
+    "Enable showing Happiness Tracking Surveys Demo to users on Desktop";
+
 const char kIntentPickerName[] = "Intent picker";
 const char kIntentPickerDescription[] =
     "When going to a site that has URL managable by a PWA, show the intent"