Add finch gate for captive portal interstitial.

chrome/browser/captive_portal/captive_portal_browsertest.cc

Index: chrome/browser/captive_portal/captive_portal_browsertest.cc
diff --git a/chrome/browser/captive_portal/captive_portal_browsertest.cc b/chrome/browser/captive_portal/captive_portal_browsertest.cc
index 1fcb08c4ee4c0b51930d8a5b1a4682fcf33a6192..d128f938b51bc9505616a2b5b76a3748245ff3d6 100644
--- a/chrome/browser/captive_portal/captive_portal_browsertest.cc
+++ b/chrome/browser/captive_portal/captive_portal_browsertest.cc
@@ -905,6 +905,7 @@ class CaptivePortalBrowserTest : public InProcessBrowserTest {
   // InProcessBrowserTest:
   void SetUpOnMainThread() override;
   void TearDownOnMainThread() override;
+  void SetUpCommandLine(base::CommandLine* command_line) override;
 
   // Sets the captive portal checking preference.  Does not affect the command
   // line flag, which is set in SetUpCommandLine.
@@ -1118,6 +1119,13 @@ void CaptivePortalBrowserTest::TearDownOnMainThread() {
   EXPECT_FALSE(CheckPending(browser()));
 }
 
+void CaptivePortalBrowserTest::SetUpCommandLine(
+    base::CommandLine* command_line) {
+  // Enable finch experiment for captive portal interstitials.
+  command_line->AppendSwitchASCII(
+      switches::kForceFieldTrials, "CaptivePortalInterstitial/Enabled/");
+}
+
 void CaptivePortalBrowserTest::EnableCaptivePortalDetection(
     Profile* profile, bool enabled) {
   profile->GetPrefs()->SetBoolean(prefs::kAlternateErrorPagesEnabled, enabled);