Add background stories to MobileSystemHealth

tools/perf/page_sets/data/system_health_mobile.json

 {
     "archives": {
         "system_health_mobile_000.wpr": [
             "load:search:amazon",
             "load:search:baidu",
             "load:search:yandex",
             "load:search:taobao",
             "load:search:ebay",
             "load:search:yahoo"
         ],
         "system_health_mobile_001.wpr": [
             "load:social:tumblr",
             "load:social:pinterest",
             "load:social:instagram"
         ],
         "system_health_mobile_002.wpr": [
             "load:news:sohu",
             "load:news:bbc",
             "load:news:wikipedia"
         ],
         "system_health_mobile_003.wpr": [
             "load:media:9gag",
             "load:media:google_images",
             "load:media:soundcloud",
             "load:media:youtube",
             "load:media:dailymotion",
             "load:media:flickr"
         ],
         "system_health_mobile_004.wpr": [
-            "load:tools:gmail",
             "load:tools:maps",
             "load:tools:stackoverflow",
             "load:tools:dropbox",
             "load:tools:weather",
             "load:tools:drive",
             "load:tools:docs"
         ],
         "system_health_mobile_005.wpr": [
             "load:games:lazors",
             "load:games:bubbles",
@@ skipping 62 matching lines @@
         "system_health_mobile_037.wpr": [
             "browse:media:youtube"
         ],
         "system_health_mobile_040.wpr": [
             "load:media:facebook_photos",
             "browse:media:facebook_photos"
         ],
         "system_health_mobile_042.wpr": [
             "long_running:tools:gmail-foreground",
             "long_running:tools:gmail-background"
+        ],
+        "system_health_mobile_043.wpr": [
+            "background:tools:gmail"
+        ],
+        "system_health_mobile_044.wpr": [
+            "background:search:google"
+        ],
+        "system_health_mobile_045.wpr": [
+            "background:social:facebook"
+        ],
+        "system_health_mobile_047.wpr": [
+            "background:media:imgur"
+        ],
+        "system_health_mobile_048.wpr": [
+            "load:tools:gmail"
+        ],
+        "system_health_mobile_052.wpr": [
+            "background:news:nytimes"
         ]
     },
     "description": "Describes the Web Page Replay archives for a story set. Don't edit by hand! Use record_wpr for updating."
-}
+}