Write skia_unittests.isolate and enable swarming for skia_unittests.

testing/buildbot/chromium.linux.json

Index: testing/buildbot/chromium.linux.json
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index 44947776b94bc66931fee7e324e466e139898ab1..bdbb8da1db7a7b66be695f925b4a6b721af037da 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -637,6 +637,9 @@
         "test": "sandbox_linux_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -903,6 +906,9 @@
         "test": "sandbox_linux_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -1153,6 +1159,9 @@
         "test": "sandbox_linux_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {