depot_tools: add infra_paths recipe module for infra-specific paths

recipe_modules/gclient/example.expected/basic.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "config",
       "--spec",
       "cache_dir = '[ROOT]/git_cache'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/chromium/src.git'}]"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient setup"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "sync",
       "--verbose",
       "--with_branch_heads",
       "--nohooks",
       "-j8",
       "--reset",
       "--force",
       "--upstream",
       "--no-nag-max",
       "--delete_unversioned_trees",
       "--output-json",
       "/path/to/tmp/json"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient sync",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"solutions\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"src/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": \"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "recurse",
       "git",
       "config",
       "user.name",
       "local_bot"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient recurse (git config user.name)"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "recurse",
       "git",
       "config",
       "user.email",
       "local_bot@example.com"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient recurse (git config user.email)"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "config",
       "--spec",
-      "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'WebKit', 'safesync_url': 'https://blink-status.appspot.com/lkgr', 'url': 'svn://svn.chromium.org/blink/trunk'}]"
+      "cache_dir = '[ROOT]/b/build/slave/cache_dir'\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'WebKit', 'safesync_url': 'https://blink-status.appspot.com/lkgr', 'url': 'svn://svn.chromium.org/blink/trunk'}]"
     ],
-    "cwd": "[SLAVE_BUILD]/src/third_party",
+    "cwd": "[CWD]/src/third_party",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "[spec: WebKit] gclient setup"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "sync",
       "--nohooks",
       "--force",
       "--verbose",
       "--delete_unversioned_trees",
       "--with_branch_heads",
       "--revision",
       "third_party/WebKit@123",
       "--output-json",
       "/path/to/tmp/json"
     ],
-    "cwd": "[SLAVE_BUILD]/src/third_party",
+    "cwd": "[CWD]/src/third_party",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "[spec: WebKit] gclient sync",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"solutions\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"WebKit/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 241198@@@",
       "@@@STEP_LOG_LINE@json.output@    }, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"src/blatley/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 248087@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@got_blatley_revision@248087@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys\n\nbuild_path = sys.argv[1]\nif os.path.exists(build_path):\n  for (path, dir, files) in os.walk(build_path):\n    for cur_file in files:\n      if cur_file.endswith('index.lock'):\n        path_to_file = os.path.join(path, cur_file)\n        print 'deleting %s' % path_to_file\n        os.remove(path_to_file)\n",
-      "[SLAVE_BUILD]"
+      "[CWD]"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "cleanup index.lock",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import os, sys@@@",
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@build_path = sys.argv[1]@@@",
       "@@@STEP_LOG_LINE@python.inline@if os.path.exists(build_path):@@@",
       "@@@STEP_LOG_LINE@python.inline@  for (path, dir, files) in os.walk(build_path):@@@",
       "@@@STEP_LOG_LINE@python.inline@    for cur_file in files:@@@",
       "@@@STEP_LOG_LINE@python.inline@      if cur_file.endswith('index.lock'):@@@",
       "@@@STEP_LOG_LINE@python.inline@        path_to_file = os.path.join(path, cur_file)@@@",
       "@@@STEP_LOG_LINE@python.inline@        print 'deleting %s' % path_to_file@@@",
       "@@@STEP_LOG_LINE@python.inline@        os.remove(path_to_file)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]