Reland "Set limit for field trial parameters length."

This is a reland of 1547c775

Original change's description:
> Set limit for field trial parameters length.
>
> Windows has a limit of 32767 on a command line. When testing with field
> trails, the command to launch chrome.exe may exceed the limit if there're
> too many field trial configs passed as parameters. This was causing
> the chrome perf waterfall to completely fail on Windows.
>
> This CL added a presubmit check to see whether the configs under Windows
> will exceed the limit of 31000.
>
> Bug: chromium:1045530
> Change-Id: I9d181783f1bdad58eeba802583c052bfd0321c6f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2021468
> Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
> Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#735981}

Bug: chromium:1045530
Change-Id: I04b664b6f8a6b21f6202df95d912bd220d669728
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2025511
Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736166}

testing/variations/PRESUBMIT.py

@@ -18,6 +18,16 @@ VALID_EXPERIMENT_KEYS = [
     '//0', '//1', '//2', '//3', '//4', '//5', '//6', '//7', '//8', '//9'
 ]
 
+FIELDTRIAL_CONFIG_FILE_NAME = 'fieldtrial_testing_config.json'
+
+FIELDTRIAL_CONFIG_TOO_LONG_ERROR_MSG = \
+  "Contents of %s result in command-line flags that exceed Windows' 32767 "\
+  "character limit. To add a new entry, please remove existing obsolete "\
+  "entries. To check if an entry is obsolete, do a code search for its "\
+  "'enable_features' and 'disable_features' strings and verify there are "\
+  "no results other than the files related to the testing config. "\
+  "See crbug.com/1045530 for more details." % FIELDTRIAL_CONFIG_FILE_NAME
+
 
 def PrettyPrint(contents):
   """Pretty prints a fieldtrial configuration.
@@ -88,10 +98,12 @@ def PrettyPrint(contents):
       ordered_config, sort_keys=False, indent=4, separators=(',', ': ')) + '\n'
 
 
-def ValidateData(json_data, file_path, message_type):
+def ValidateData(input_api, json_data, file_path, message_type):
   """Validates the format of a fieldtrial configuration.
 
   Args:
+    input_api: An instance passed to presubmit scripts telling info about the
+      changes.
     json_data: Parsed JSON object representing the fieldtrial config.
     file_path: String representing the path to the JSON file.
     message_type: Type of message from |output_api| to return in the case of
@@ -106,6 +118,9 @@ def ValidateData(json_data, file_path, message_type):
     return _CreateMalformedConfigMessage(message_type, file_path,
                                          message_format, *args)
 
+  if not _IsFieldTrialSizeBelowLimitOnWindows(input_api, file_path):
+    return [message_type(FIELDTRIAL_CONFIG_TOO_LONG_ERROR_MSG)]
+
   if not isinstance(json_data, dict):
     return _CreateMessage('Expecting dict')
   for (study, experiment_configs) in json_data.iteritems():
@@ -228,15 +243,54 @@ def CheckPretty(contents, file_path, message_type):
   return []
 
 
+def _IsFieldTrialSizeBelowLimitOnWindows(input_api, file_path):
+  """Checks whether the fieldtrial parameters exceeded the Windows cmd limit.
+
+  When launching chrome, the fieldtrial related parameters take more than
+  30,000 characters. Windows has a limit of 32767 characters on command
+  line and thus it raises parameter error when the parameters are too long.
+  Before we have a valid fix, we need to limit the number of fieldtrias in
+  fieldtrial_testing_config.json to 31,500, from the fact that the
+  non-fieldtrial parameters take roughly 1450 characters.
+  See crbug.com/1045530 for more details.
+
+  Args:
+    input_api: An instance passed to presubmit scripts telling info about the
+      changes.
+    file_path: the absolute path to the json file with the fieldtrial configs.
+  """
+  sys.path.append(input_api.os_path.join(
+      input_api.PresubmitLocalPath(), '..', '..', 'tools', 'variations'))
+  import fieldtrial_util
+
+  args = fieldtrial_util.GenerateArgs(file_path, 'windows')
+  total_length = 0
+  for arg in args:
+    total_length += len(arg)
+  return total_length < 31500
+
+
 def CommonChecks(input_api, output_api):
   affected_files = input_api.AffectedFiles(
       include_deletes=False,
       file_filter=lambda x: x.LocalPath().endswith('.json'))
   for f in affected_files:
+    if not f.LocalPath().endswith(FIELDTRIAL_CONFIG_FILE_NAME):
+      return [
+          output_api.PresubmitError(
+              '%s is the only json file expected in this folder. If new jsons '
+              'are added, please update the presubmit process with proper '
+              'validation. ' % FIELDTRIAL_CONFIG_FILE_NAME
+          )
+      ]
     contents = input_api.ReadFile(f)
     try:
       json_data = input_api.json.loads(contents)
-      result = ValidateData(json_data, f.LocalPath(), output_api.PresubmitError)
+      result = ValidateData(
+          input_api,
+          json_data,
+          f.AbsoluteLocalPath(),
+          output_api.PresubmitError)
       if len(result):
         return result
       result = CheckPretty(contents, f.LocalPath(), output_api.PresubmitError)