Reland "Changed committed-interstitials flag from switch to feature."

This is a reland of 49040ae795592d3b0c6a5b03533565caeb9ae266

Original change's description:
> Changed committed-interstitials flag from switch to feature.
> 
> Removed 'committed-interstitials' switch and added SSLCommittedInterstitials
> base::Feature. Name was changed to SSLCommmittedInterstitials since we are now
> doing separate rampups for each kind of committed interstitial. Switch was
> changed to Feature so that it can be enabled via variations.
> 
> Bug: 755632
> Change-Id: I0f3e0e01a94be2b515aa596827cd447d14c24b56
> Reviewed-on: https://chromium-review.googlesource.com/1118975
> Commit-Queue: Carlos IL <[email protected]>
> Reviewed-by: Jialiu Lin <[email protected]>
> Reviewed-by: Maksim Ivanov <[email protected]>
> Reviewed-by: Emily Stark <[email protected]>
> Reviewed-by: Giovanni Ortuño Urquidi <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#574621}

Bug: 755632
Change-Id: I9ef1fa141d9481537740569c4af6504998c619e2
Reviewed-on: https://chromium-review.googlesource.com/1138516
Reviewed-by: Mustafa Emre Acer <[email protected]>
Reviewed-by: Maksim Ivanov <[email protected]>
Reviewed-by: Giovanni Ortuño Urquidi <[email protected]>
Reviewed-by: Jialiu Lin <[email protected]>
Commit-Queue: Carlos IL <[email protected]>
Cr-Commit-Position: refs/heads/master@{#580565}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 4736202..9714ac1 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2115,9 +2115,10 @@
      flag_descriptions::kExperimentalSecurityFeaturesDescription, kOsAll,
      SINGLE_VALUE_TYPE(switches::kEnablePotentiallyAnnoyingSecurityFeatures)},
 #endif  // OS_CHROMEOS
-    {"committed-interstitials", flag_descriptions::kCommittedInterstitialsName,
-     flag_descriptions::kCommittedInterstitialsDescription, kOsAll,
-     SINGLE_VALUE_TYPE(switches::kCommittedInterstitials)},
+    {"ssl-committed-interstitials",
+     flag_descriptions::kSSLCommittedInterstitialsName,
+     flag_descriptions::kSSLCommittedInterstitialsDescription, kOsAll,
+     FEATURE_VALUE_TYPE(features::kSSLCommittedInterstitials)},
     {"enable-site-per-process", flag_descriptions::kStrictSiteIsolationName,
      flag_descriptions::kStrictSiteIsolationDescription, kOsAll,
      SINGLE_VALUE_TYPE(switches::kSitePerProcess)},