Write skia_unittests.isolate and enable swarming for skia_unittests.

testing/buildbot/chromium.win.json

Index: testing/buildbot/chromium.win.json
diff --git a/testing/buildbot/chromium.win.json b/testing/buildbot/chromium.win.json
index d32c10d3a4077d431698fa8d54cd522cbfd4acda..0bb7375a921e64bfb02e167d1d079d9cb33483cc 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -190,6 +190,9 @@
         "test": "sbox_validation_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -452,6 +455,9 @@
         "test": "sbox_validation_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -833,6 +839,9 @@
         "test": "sbox_validation_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -1073,6 +1082,9 @@
         "test": "sbox_validation_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {
@@ -1473,6 +1485,9 @@
         "test": "sbox_validation_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "skia_unittests"
       },
       {