Stop to use contextmanager for goma.build_with_goma

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_linux_arm.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]/linux_arm",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 179 matching lines @@
       "--gyp-script=[BUILDER_CACHE]/linux_arm/src/webrtc/build/gyp_webrtc.py",
       "//out/Release"
     ],
     "cwd": "[BUILDER_CACHE]/linux_arm/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
   },
   {
-    "cmd": [
-      "python",
-      "-u",
-      "\nimport multiprocessing\nimport sys\n\njob_limit = 200\nif sys.platform.startswith('linux'):\n  # Use 80 for linux not to load goma backend.\n  job_limit = 80\n\ntry:\n  jobs = min(job_limit, multiprocessing.cpu_count() * 10)\nexcept NotImplementedError:\n  jobs = 50\n\nprint jobs\n"
-    ],
-    "name": "calculate the number of recommended jobs",
-    "stdout": "/path/to/tmp/",
-    "~followup_annotations": [
-      "@@@STEP_LOG_LINE@python.inline@@@@",
-      "@@@STEP_LOG_LINE@python.inline@import multiprocessing@@@",
-      "@@@STEP_LOG_LINE@python.inline@import sys@@@",
-      "@@@STEP_LOG_LINE@python.inline@@@@",
-      "@@@STEP_LOG_LINE@python.inline@job_limit = 200@@@",
-      "@@@STEP_LOG_LINE@python.inline@if sys.platform.startswith('linux'):@@@",
-      "@@@STEP_LOG_LINE@python.inline@  # Use 80 for linux not to load goma backend.@@@",
-      "@@@STEP_LOG_LINE@python.inline@  job_limit = 80@@@",
-      "@@@STEP_LOG_LINE@python.inline@@@@",
-      "@@@STEP_LOG_LINE@python.inline@try:@@@",
-      "@@@STEP_LOG_LINE@python.inline@  jobs = min(job_limit, multiprocessing.cpu_count() * 10)@@@",
-      "@@@STEP_LOG_LINE@python.inline@except NotImplementedError:@@@",
-      "@@@STEP_LOG_LINE@python.inline@  jobs = 50@@@",
-      "@@@STEP_LOG_LINE@python.inline@@@@",
-      "@@@STEP_LOG_LINE@python.inline@print jobs@@@",
-      "@@@STEP_LOG_END@python.inline@@@"
-    ]
-  },
-  {
     "cmd": [],
     "name": "preprocess_for_goma"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[CACHE]/cipd/goma/goma_ctl.py",
       "restart"
     ],
@@ skipping 22 matching lines @@
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "preprocess_for_goma.start cloudtail",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
   },
   {
     "cmd": [
+      "python",
+      "-u",
+      "\nimport multiprocessing\nimport sys\n\njob_limit = 200\nif sys.platform.startswith('linux'):\n  # Use 80 for linux not to load goma backend.\n  job_limit = 80\n\ntry:\n  jobs = min(job_limit, multiprocessing.cpu_count() * 10)\nexcept NotImplementedError:\n  jobs = 50\n\nprint jobs\n"
+    ],
+    "name": "calculate the number of recommended jobs",
+    "stdout": "/path/to/tmp/",
+    "~followup_annotations": [
+      "@@@STEP_LOG_LINE@python.inline@@@@",
+      "@@@STEP_LOG_LINE@python.inline@import multiprocessing@@@",
+      "@@@STEP_LOG_LINE@python.inline@import sys@@@",
+      "@@@STEP_LOG_LINE@python.inline@@@@",
+      "@@@STEP_LOG_LINE@python.inline@job_limit = 200@@@",
+      "@@@STEP_LOG_LINE@python.inline@if sys.platform.startswith('linux'):@@@",
+      "@@@STEP_LOG_LINE@python.inline@  # Use 80 for linux not to load goma backend.@@@",
+      "@@@STEP_LOG_LINE@python.inline@  job_limit = 80@@@",
+      "@@@STEP_LOG_LINE@python.inline@@@@",
+      "@@@STEP_LOG_LINE@python.inline@try:@@@",
+      "@@@STEP_LOG_LINE@python.inline@  jobs = min(job_limit, multiprocessing.cpu_count() * 10)@@@",
+      "@@@STEP_LOG_LINE@python.inline@except NotImplementedError:@@@",
+      "@@@STEP_LOG_LINE@python.inline@  jobs = 50@@@",
+      "@@@STEP_LOG_LINE@python.inline@@@@",
+      "@@@STEP_LOG_LINE@python.inline@print jobs@@@",
+      "@@@STEP_LOG_END@python.inline@@@"
+    ]
+  },
+  {
+    "cmd": [
       "RECIPE_PACKAGE_REPO[depot_tools]/ninja",
       "-w",
       "dupbuild=err",
       "-C",
       "[BUILDER_CACHE]/linux_arm/src/out/Release",
       "-j",
       "50"
     ],
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
@@ skipping 94 matching lines @@
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]