Reland "Skia: Maintain a checkout on swarming (compile) bots"

scripts/slave/recipes/skia/swarm_trigger.expected/Build-Win8-MSVC-x86_64-Release-Swarming.json

 [
   {
     "cmd": [
       "git",
       "remote",
       "set-url",
       "origin",
       "https://skia.googlesource.com/skia.git"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
@@ skipping 412 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  \"compile_skia\": \"[dummy hash for compile_skia]\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@swarm_hashes@{\"compile_skia\": \"[dummy hash for compile_skia]\"}@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
-      "\nimport json\nimport sys\nwith open(sys.argv[1]) as f:\n  isolated = json.load(f)\nfor h in sys.argv[2:]:\n  isolated['includes'].append(h)\nwith open(sys.argv[1], 'w') as f:\n  json.dump(isolated, f, sort_keys=True)\n",
+      "\nimport json\nimport sys\nwith open(sys.argv[1]) as f:\n  isolated = json.load(f)\nif not isolated.get('includes'):\n  isolated['includes'] = []\nfor h in sys.argv[2:]:\n  isolated['includes'].append(h)\nwith open(sys.argv[1], 'w') as f:\n  json.dump(isolated, f, sort_keys=True)\n",
       "[SLAVE_BUILD]/swarming_temp_dir/skia-task-compile_skia.isolated",
       "[dummy hash for isolate_recipes]",
       "705384d88f80da637eb367e5acc6f315c0e1db2f"
     ],
     "name": "add_isolated_input",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import json@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys@@@",
       "@@@STEP_LOG_LINE@python.inline@with open(sys.argv[1]) as f:@@@",
       "@@@STEP_LOG_LINE@python.inline@  isolated = json.load(f)@@@",
+      "@@@STEP_LOG_LINE@python.inline@if not isolated.get('includes'):@@@",
+      "@@@STEP_LOG_LINE@python.inline@  isolated['includes'] = []@@@",
       "@@@STEP_LOG_LINE@python.inline@for h in sys.argv[2:]:@@@",
       "@@@STEP_LOG_LINE@python.inline@  isolated['includes'].append(h)@@@",
       "@@@STEP_LOG_LINE@python.inline@with open(sys.argv[1], 'w') as f:@@@",
       "@@@STEP_LOG_LINE@python.inline@  json.dump(isolated, f, sort_keys=True)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "cmd": [
       "python",
@@ skipping 145 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]