Isolate extensions_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 91c71ef071d2d3a6cc03a8d661bf2389a44db17f..78c85087e4c84f9f43ceeeb057ce123cf915ccf4 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -96,6 +96,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -370,6 +373,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -772,6 +778,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -1055,6 +1064,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -1479,6 +1491,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {