Update the BetterMultiTabsLoading experiment

Don't enable the "StaggeredBackgroundTabOpening*" features in this
experiment, they are independent now.

Bug: 829933
Change-Id: Ie30c540974773cc7f9dc53f764766f05aa0e1d14
Reviewed-on: https://chromium-review.googlesource.com/1136652Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575064}

testing/variations/fieldtrial_testing_config.json

@@ -720,17 +720,12 @@
             ],
             "experiments": [
                 {
-                    "name": "PageAlmostIdleSignalEnabled",
+                    "name": "PageAlmostIdleSignalEnabled2",
                     "params": {
                         "mainThreadTaskLoadLowThreshold": "25"
                     },
                     "enable_features": [
-                        "PageAlmostIdle",
-                        "StaggeredBackgroundTabOpening",
-                        "StaggeredBackgroundTabOpeningExperiment"
-                    ],
-                    "disable_features": [
-                        "CustomizedTabLoadTimeout"
+                        "PageAlmostIdle"
                     ]
                 }
             ]