[base] Re-enable accidently disabled BlinkMainThreadUsesSequenceManager feature

Accidently disabled as part of crrev.com/627191 and it caused a ton
of regressions (see bugs dupped into crbug.com/927619). So we should
probably ship this configuration by default, but as an emergency
bandaid I'm reenabling the feature in tests (and this CL will also
conveniently let us know of any regressions when reenabling should
disabling have improved other metrics..!).

R=skyostil@chromium.org
TBR=isherman@chromium.org

Bug: 927619, 891670, 885371
Change-Id: I9b737a786da5e589ad816827e8d52d439e785204
Reviewed-on: https://chromium-review.googlesource.com/c/1449575Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628502}

testing/variations/fieldtrial_testing_config.json

@@ -4273,6 +4273,26 @@
             ]
         }
     ],
+    "SequenceManagerRollout": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "ios",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "BlinkMainThreadUsesSequenceManager"
+                    ]
+                }
+            ]
+        }
+    ],
     "ServiceWorkerServicification": [
         {
             "platforms": [