[milo] remove linkBase and calculate links directly.

milo/buildsource/swarming/expectations/build-nested.json

 {
   "Summary": {
     "Label": "2d8d62f6be545b10",
     "Status": "Running",
     "Source": {
       "Label": "Task 2d8d62f6be545b10",
       "URL": "https://example.com/task?id=2d8d62f6be545b10\u0026show_raw=1\u0026wide_logs=true"
     },
     "MainLink": [
       {
@@ skipping 14 matching lines @@
     ]
   },
   "SourceStamp": null,
   "Components": [
     {
       "Label": "recipe bootstrap",
       "Status": "Running",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/recipe_bootstrap/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/recipe_bootstrap/0/stdout"
         }
       ],
       "Started": "2016-11-14T22:36:06Z",
       "Finished": "0001-01-01T00:00:00Z",
       "Duration": 0,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": null
     },
     {
       "Label": "setup_build",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/setup_build/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/setup_build/0/stdout"
         }
       ],
       "SubLink": [
         [
           {
             "Label": "run_recipe",
-            "URL": "foo/steps/setup_build/0/logs/run_recipe/0"
+            "URL": "/swarming/task/build-nested/steps/setup_build/0/logs/run_recipe/0"
           }
         ]
       ],
       "Started": "2016-11-14T22:36:12.970000128Z",
       "Finished": "2016-11-14T22:36:13.02Z",
       "Duration": 49999872,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 1,
       "Text": [
         "running recipe: \"magenta\""
       ]
     },
     {
       "Label": "install cipd",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/install_cipd/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/install_cipd/0/stdout"
         }
       ],
       "SubLink": [
         [
           {
             "Label": "json.output",
-            "URL": "foo/steps/install_cipd/0/logs/json.output/0"
+            "URL": "/swarming/task/build-nested/steps/install_cipd/0/logs/json.output/0"
           }
         ]
       ],
       "Started": "2016-11-14T22:36:13.02Z",
       "Finished": "2016-11-14T22:36:13.450000128Z",
       "Duration": 430000128,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": [
         "cipd instance_id: d1ec9d904caee34beffec0a691a79991122037ba"
       ]
     },
     {
       "Label": "ensure_installed",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/ensure_installed/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/ensure_installed/0/stdout"
         }
       ],
       "SubLink": [
         [
           {
             "Label": "json.output",
-            "URL": "foo/steps/ensure_installed/0/logs/json.output/0"
+            "URL": "/swarming/task/build-nested/steps/ensure_installed/0/logs/json.output/0"
           }
         ]
       ],
       "Started": "2016-11-14T22:36:13.450000128Z",
       "Finished": "2016-11-14T22:36:13.68Z",
       "Duration": 229999872,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": null
     },
     {
       "Label": "jiri init",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/jiri_init/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/jiri_init/0/stdout"
         }
       ],
       "Started": "2016-11-14T22:36:13.68Z",
       "Finished": "2016-11-14T22:36:14.06Z",
       "Duration": 380000000,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": null
     },
     {
       "Label": "jiri project clean",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/jiri_project_clean/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/jiri_project_clean/0/stdout"
         }
       ],
       "Started": "2016-11-14T22:36:14.06Z",
       "Finished": "2016-11-14T22:36:14.089999872Z",
       "Duration": 29999872,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": null
     },
     {
       "Label": "jiri import",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/jiri_import/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/jiri_import/0/stdout"
         }
       ],
       "Started": "2016-11-14T22:36:14.089999872Z",
       "Finished": "2016-11-14T22:36:14.089999872Z",
       "Duration": 0,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": null
     },
     {
       "Label": "jiri update",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/jiri_update/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/jiri_update/0/stdout"
         }
       ],
       "Started": "2016-11-14T22:36:14.089999872Z",
       "Finished": "2016-11-14T22:37:14.029999872Z",
       "Duration": 59940000000,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": null
     },
     {
       "Label": "build",
       "Status": "Failure",
       "Started": "2016-11-14T22:37:58.5Z",
       "Finished": "2016-11-14T22:37:58.5Z",
       "Duration": 0,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 2,
       "Text": null
     },
     {
       "Label": "build.cleanup",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/build/0/steps/cleanup/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/build/0/steps/cleanup/0/stdout"
         }
       ],
       "Started": "2016-11-14T22:37:14.78Z",
       "Finished": "2016-11-14T22:37:16.329999872Z",
       "Duration": 1549999872,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 0,
       "Text": null
     },
     {
       "Label": "build.build",
       "Status": "Failure",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/build/0/steps/build/0/stdout"
+          "URL": "/swarming/task/build-nested/steps/build/0/steps/build/0/stdout"
         }
       ],
       "Started": "2016-11-14T22:37:16.329999872Z",
       "Finished": "2016-11-14T22:37:58.5Z",
       "Duration": 42170000128,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 2,
       "Text": null
     }
@@ skipping 43 matching lines @@
       "GroupName": "build.build",
       "Property": null
     },
     {
       "GroupName": "Main",
       "Property": null
     }
   ],
   "Blame": null
 }