Add seek tests to media constrained network benchmark.

tools/perf/page_sets/media_cns_cases.json

 {
   "description": "Media benchmark on network constrained conditions.",
   "media_metrics": {"action": "play-action"},
   "add_browser_metrics": true,
   "play-action": [
     { "action": "play",
       "wait_for_playing": true,
       "wait_for_ended": true
     }
   ],
+  "seek_before_and_after_playhead": [
+    { "action": "play",
+      "wait_for_playing": true,
+      "wait_for_ended": false
+    },
+    { "_comment": "Wait for 1 second so that we know the play-head is at ~1s.",
+      "action": "wait", "seconds": 1
+    },
+    { "_comment": "Seek to before the play-head location.",
+      "action": "seek",
+      "wait_for_seeked": true,
+      "seek_time": "0.5",
+      "seek_label": "seek_warm"
+    },
+    { "_comment": "Seek to after the play-head location.",
+      "action": "seek",
+      "wait_for_seeked": true,
+      "seek_time": 15,
+      "seek_label": "seek_cold"
+    }
+  ],
   "pages": [
     {
       "url": "file://tough_video_cases/video.html?id=no_constraints_webm&src=tulip2.webm&net=none"
     },
     {
       "url": "file://tough_video_cases/video.html?id=cable_webm&src=tulip2.webm&net=cable"
     },
     {
       "url": "file://tough_video_cases/video.html?id=wifi_webm&src=tulip2.webm&net=wifi"
     },
@@ skipping 43 matching lines @@
       "url": "file://tough_video_cases/video.html?id=wifi_mp3&src=tulip2.mp3&type=audio&net=wifi"
     },
     {
       "url": "file://tough_video_cases/video.html?id=no_constraints_m4a&src=tulip2.m4a&type=audio&net=none"
     },
     {
       "url": "file://tough_video_cases/video.html?id=cable_m4a&src=tulip2.m4a&type=audio&net=cable"
     },
     {
       "url": "file://tough_video_cases/video.html?id=wifi_m4a&src=tulip2.m4a&type=audio&net=wifi"
+    },
+    {
+      "url": "file://tough_video_cases/video.html?id=wifi_mp3&src=tulip2.mp3&type=audio&net=wifi",
+      "skip_basic_metrics": true,
+      "add_browser_metrics": false,
+      "media_metrics": {"action": "seek_before_and_after_playhead"}
+    },
+    {
+      "url": "file://tough_video_cases/video.html?id=wifi_m4a&src=tulip2.m4a&type=audio&net=wifi",
+      "skip_basic_metrics": true,
+      "add_browser_metrics": false,
+      "media_metrics": {"action": "seek_before_and_after_playhead"}
+    },
+    {
+      "url": "file://tough_video_cases/video.html?id=wifi_ogg&src=tulip2.ogg&type=audio&net=wifi",
+      "skip_basic_metrics": true,
+      "add_browser_metrics": false,
+      "media_metrics": {"action": "seek_before_and_after_playhead"}
+    },
+    {
+      "url": "file://tough_video_cases/video.html?id=wifi_wav&src=tulip2.wav&type=audio&net=wifi",
+      "skip_basic_metrics": true,
+      "add_browser_metrics": false,
+      "media_metrics": {"action": "seek_before_and_after_playhead"}
+    },
+    {
+      "url": "file://tough_video_cases/video.html?id=wifi_mp4&src=tulip2.mp4&type=audio&net=wifi",
+      "skip_basic_metrics": true,
+      "add_browser_metrics": false,
+      "media_metrics": {"action": "seek_before_and_after_playhead"}
+    },
+    {
+      "url": "file://tough_video_cases/video.html?id=wifi_ogv&src=tulip2.ogv&type=audio&net=wifi",
+      "skip_basic_metrics": true,
+      "add_browser_metrics": false,
+      "media_metrics": {"action": "seek_before_and_after_playhead"}
+    },
+    {
+      "url": "file://tough_video_cases/video.html?id=wifi_webm&src=tulip2.webm&type=audio&net=wifi",
+      "skip_basic_metrics": true,
+      "add_browser_metrics": false,
+      "media_metrics": {"action": "seek_before_and_after_playhead"}
     }
   ]
 }