[system health] Add media browsing stories.

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:google",
             "load:search:taobao",
             "load:search:ebay",
             "load:search:yahoo"
@@ skipping 76 matching lines @@
             "load:news:flipboard"
         ],
         "system_health_mobile_025.wpr": [
             "browse:news:flipboard"
         ],
         "system_health_mobile_027.wpr": [
             "browse:news:reddit"
         ],
         "system_health_mobile_028.wpr": [
             "browse:social:facebook"
+        ],
+        "system_health_mobile_035.wpr": [
+            "browse:media:imgur",
+            "load:media:imgur"
+        ],
+        "system_health_mobile_037.wpr": [
+            "browse:media:youtube"
+        ],
+        "system_health_mobile_038.wpr": [
+            "browse:media:facebookphotos"
         ]
     },
     "description": "Describes the Web Page Replay archives for a story set. Don't edit by hand! Use record_wpr for updating."
-}
+}