Skip interstitials and don't block requests for localhost SSL errors

For now this behavior is behind a flag, because there are some weird
cases where localhost certificate errors could indicate a real security
problem (see https://code.google.com/p/chromium/issues/detail?id=282927
for more detail).

BUG=282927
TEST=Set up HTTPS server on localhost with a bad certificate, visit
https://localhost and observe no interstitial

Review URL: https://codereview.chromium.org/887223005

Cr-Commit-Position: refs/heads/master@{#316347}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 4fc9e14..4685d1e1 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2181,6 +2181,13 @@
                       kEnableDataReductionProxyBypassWarning)
   },
 #endif
+  {
+    "allow-insecure-localhost",
+    IDS_ALLOW_INSECURE_LOCALHOST,
+    IDS_ALLOW_INSECURE_LOCALHOST_DESCRIPTION,
+    kOsAll,
+    SINGLE_VALUE_TYPE(switches::kAllowInsecureLocalhost)
+  },
 
   // NOTE: Adding new command-line switches requires adding corresponding
   // entries to enum "LoginCustomFlags" in histograms.xml. See note in