Add perf histogram and tests for inline signin

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 9dbb372e899c8ca02b4d44867d031e5c83fa45e6..c31c165a78856187d9c303dde83a699a40ab4776 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -886,6 +886,9 @@ const char kForceAppMode[]                  = "force-app-mode";
 // kCancelFirstRun).
 const char kForceFirstRun[]                 = "force-first-run";
 
+// Forces showing the signin promo on startup.
+const char kForceSigninPromo[]              = "force-signin-promo";
+
 // Forces additional Chrome Variation Ids that will be sent in
 // X-Client-Data header, specified as a 64-bit encoded list of numeric
 // experiment ids.