Run gtest tests on recipe "WebKit Linux" builder

BUG=460949
R=primiano@chromium.org

Review URL: https://codereview.chromium.org/972673002

Cr-Commit-Position: refs/heads/master@{#318699}

testing/buildbot/chromium.webkit.json

 {
   "WebKit Linux": {
+    "gtest_tests": [
+      "blink_heap_unittests",
+      "blink_platform_unittests",
+      "webkit_unit_tests",
+      "wtf_unittests"
+    ],
     "scripts": [
       {
         "name": "webkit_lint",