Make CACHE_DIR set by default in most gclient configs.

recipe_modules/gclient/example.expected/tryserver.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE[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'}]"
     ],
@@ skipping 73 matching lines @@
     },
     "name": "gclient recurse (git config user.email)"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE[depot_tools]/gclient.py",
       "config",
       "--spec",
-      "cache_dir = None\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 = '[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'}]"
     ],
     "cwd": "[SLAVE_BUILD]/src/third_party",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE[depot_tools]"
     },
     "name": "[spec: WebKit] gclient setup"
   },
   {
     "cmd": [
       "python",
@@ skipping 77 matching lines @@
       "PATH": "%(PATH)s:RECIPE_PACKAGE[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]