Revert "Enable passive mixed content warning by default"

This reverts commit 0a37869a.

Reason for revert: Causing failure of DidChangeVisibleSecurityStateTestWithAutoupgradesDisabled.DidChangeVisibleSecurityStateObserver in browser_tests on linux-chromeos-chrome
Example failure: https://ci.chromium.org/p/chrome/builders/ci/linux-chromeos-chrome/3194

Original change's description:
> Enable passive mixed content warning by default
> 
> Sets the feature to enabled by default and removes the config from
> fieldtrial_testing_config.json in preparation for launch.
> 
> Bug: 1025276
> Change-Id: Iffcdd2153491de25b7038a609776255bbdacf2ac
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053024
> Commit-Queue: Carlos IL <carlosil@chromium.org>
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Reviewed-by: Emily Stark <estark@chromium.org>
> Auto-Submit: Carlos IL <carlosil@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#741506}

TBR=rkaplow@chromium.org,estark@chromium.org,carlosil@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1025276
Change-Id: I20e9dc38d83036c7b17d91de24c23ebae3049de9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2058970Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741860}

components/security_state/core/features.cc

@@ -22,7 +22,7 @@ const base::Feature kSafetyTipUI{"SafetyTip",
                                  base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kPassiveMixedContentWarning{
-    "PassiveMixedContentWarning", base::FEATURE_ENABLED_BY_DEFAULT};
+    "PassiveMixedContentWarning", base::FEATURE_DISABLED_BY_DEFAULT};
 
 }  // namespace features
 }  // namespace security_state

components/security_state/core/security_state_unittest.cc

@@ -303,11 +303,7 @@ TEST(SecurityStateTest, MixedContentWithPolicyCertificate) {
   // Verify that passive mixed content downgrades the security level.
   helper.set_contained_mixed_form(false);
   helper.set_displayed_mixed_content(true);
-  SecurityLevel expected_passive_level =
-      base::FeatureList::IsEnabled(features::kPassiveMixedContentWarning)
-          ? WARNING
-          : NONE;
-  EXPECT_EQ(expected_passive_level, helper.GetSecurityLevel());
+  EXPECT_EQ(NONE, helper.GetSecurityLevel());
 
   // Ensure that active mixed content downgrades the security level.
   helper.set_contained_mixed_form(false);

testing/variations/fieldtrial_testing_config.json

@@ -4736,6 +4736,25 @@
             ]
         }
     ],
+    "PassiveMixedContentWarning": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "PassiveMixedContentWarning"
+                    ]
+                }
+            ]
+        }
+    ],
     "PasswordManagerStickyBubble": [
         {
             "platforms": [