Reland: Use real revision mapping in bot_update module

scripts/slave/recipes/android_webview_aosp.expected/basic.json

 [
   {
     "abort_on_failure": true,
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.testing.master",
       "--builder",
       "TestBuilder",
       "--slave",
       "TestSlavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_vars': {'googlecode_url': 'svn://svn-mirror.golo.chromium.org/%s', 'nacl_trunk': 'svn://svn-mirror.golo.chromium.org/native_client/trunk', 'sourceforge_url': 'svn://svn-mirror.golo.chromium.org/%(repo)s', 'webkit_trunk': 'svn://svn-mirror.golo.chromium.org/blink/trunk'}, 'deps_file': '', 'managed': True, 'name': 'src', 'url': 'svn://svn-mirror.golo.chromium.org/chrome/trunk/src'}]",
       "--revision",
       "204787",
+      "--revision_mapping_file",
+      "{}",
       "--output_json",
       "/path/to/tmp/json"
     ],
     "name": "bot_update - empty_deps",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": false@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
@@ skipping 92 matching lines @@
       "--master",
       "chromium.testing.master",
       "--builder",
       "TestBuilder",
       "--slave",
       "TestSlavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_deps': {'src/blacklist/project/1': None, 'src/blacklist/project/2': None}, 'custom_vars': {'googlecode_url': 'svn://svn-mirror.golo.chromium.org/%s', 'nacl_trunk': 'svn://svn-mirror.golo.chromium.org/native_client/trunk', 'sourceforge_url': 'svn://svn-mirror.golo.chromium.org/%(repo)s', 'webkit_trunk': 'svn://svn-mirror.golo.chromium.org/blink/trunk'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn-mirror.golo.chromium.org/chrome/trunk/src'}]\ntarget_os = ['android']",
       "--revision",
       "204787",
+      "--revision_mapping_file",
+      "{}",
       "--output_json",
       "/path/to/tmp/json"
     ],
     "name": "bot_update - trimmed",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": false@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
@@ skipping 222 matching lines @@
       "--src-dir",
       "[SLAVE_BUILD]/android-src",
       "--build-tool",
       "make-android",
       "--verbose"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "compile"
   }
 ]