depot_tools: add infra_paths recipe module for infra-specific paths

recipe_modules/tryserver/example.expected/with_git_patch.json

Index: recipe_modules/tryserver/example.expected/with_git_patch.json
diff --git a/recipe_modules/tryserver/example.expected/with_git_patch.json b/recipe_modules/tryserver/example.expected/with_git_patch.json
index 117f1c2e17844e6ba842e4e4258d041af898466d..a6c8fb1d34300ef7a5318297544d8a5f09e93a44 100644
--- a/recipe_modules/tryserver/example.expected/with_git_patch.json
+++ b/recipe_modules/tryserver/example.expected/with_git_patch.json
@@ -3,13 +3,12 @@
     "cmd": [
       "python",
       "-u",
-      "[BUILD]/scripts/slave/git_setup.py",
+      "[ROOT]/b/build/scripts/slave/git_setup.py",
       "--path",
       "[TMP_BASE]/patch_tmp_1",
       "--url",
       "http://patch.url/"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "patch git setup"
   },
   {
@@ -47,7 +46,7 @@
     "cmd": [
       "patch",
       "--dir",
-      "[SLAVE_BUILD]",
+      "[CWD]",
       "--force",
       "--forward",
       "--remove-empty-files",
@@ -56,7 +55,6 @@
       "--input",
       "[TMP_BASE]/patch_tmp_1/patch.diff"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "apply patch"
   },
   {
@@ -65,7 +63,6 @@
       "-rf",
       "[TMP_BASE]/patch_tmp_1"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "remove patch"
   },
   {
@@ -75,7 +72,7 @@
       "--cached",
       "--name-only"
     ],
-    "cwd": "[SLAVE_BUILD]/v8",
+    "cwd": "[CWD]/v8",
     "name": "git diff to analyze patch",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
@@ -91,7 +88,6 @@
       "-u",
       "import sys; sys.exit(1)"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "fail",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",