Reland "skia: Add 'skia_unittests' test suite for this directory."

testing/buildbot/chromium_trybot.json

 {
   "compile_targets": [
     "all"
   ],
   "gtest_tests": [
     {
       "test": "accessibility_unittests",
       "swarming": {
         "can_use_on_swarming_builders": true
       }
@@ skipping 193 matching lines @@
     {
       "test": "ui_chromeos_unittests",
       "platforms": ["linux"],
       "chromium_configs": [
         "chromium_chromeos",
         "chromium_chromeos_clang",
         "chromium_chromeos_ozone"
       ]
     },
     "url_unittests",
+    "skia_unittests",
     {
       "test": "wm_unittests",
       "platforms": ["linux", "win"]
     },
     {
       "test": "app_shell_browsertests",
       "platforms": ["linux", "win"]
     },
     {
       "test": "app_shell_unittests",
@@ skipping 51 matching lines @@
   "non_filter_tests_builders": [
   ],
 
   "filter_compile_builders": [
     "win_chromium_compile_dbg",
     "win_chromium_rel_swarming",
     "win_chromium_x64_rel_swarming",
     "win8_chromium_rel"
   ]
 }