Write skia_unittests.isolate and enable swarming for skia_unittests.

testing/buildbot/chromium.mac.json

Index: testing/buildbot/chromium.mac.json
diff --git a/testing/buildbot/chromium.mac.json b/testing/buildbot/chromium.mac.json
index 4a5ffab86db773387e4379407be5c6836d4aef47..1cb590db3981c5f3c87bd57ff849875a0b5f9c15 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -189,6 +189,9 @@
         "test": "remoting_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -391,6 +394,9 @@
         "test": "sandbox_mac_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -599,6 +605,9 @@
         "test": "sandbox_mac_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -807,6 +816,9 @@
         "test": "sandbox_mac_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -1016,6 +1028,9 @@
         "test": "sandbox_mac_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {