Enable the HeapMojoUseContextObserver feature for waterfall testing

To deprecate kWithoutContextObserver mode, we are going to enable
HeapMojoUseContextObserver feature for waterfall testing. By doing
this, we can detect crashes during the waterfall testing.

Bug: 1049056
Change-Id: I96e0be25ad6e6cc9eeac2fad600f1dadec02ecd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277369Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784344}

testing/variations/fieldtrial_testing_config.json

@@ -3155,6 +3155,27 @@
             ]
         }
     ],
+    "HeapMojoUseContextObserver": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "android_weblayer",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "HeapMojoUseContextObserver"
+                    ]
+                }
+            ]
+        }
+    ],
     "HeapProfiling": [
         {
             "platforms": [