[style_var_gen] Add override points for google3 integration.

This CL adds easy file path override points for changes that need to be
made to bring the Style Variable Generator into google3.

Bug: 1018654
Change-Id: I4b8fd9de2baeaf7972c9319a05551df32c26b50f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2198702Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770008}

tools/style_variable_generator/base_generator.py

@@ -7,6 +7,7 @@ import sys
 import collections
 import re
 import textwrap
+import path_overrides
 from color import Color
 
 _FILE_PATH = os.path.dirname(os.path.realpath(__file__))
@@ -89,13 +90,14 @@ class BaseGenerator:
             raise ValueError('\n%s:\n    %s' % (path, err))
 
     def ApplyTemplate(self, style_generator, path_to_template, params):
-        current_dir = os.path.dirname(os.path.realpath(__file__))
+        loader_root_dir = path_overrides.GetFileSystemLoaderRootDirectory()
         jinja_env = jinja2.Environment(
-            loader=jinja2.FileSystemLoader(current_dir),
+            loader=jinja2.FileSystemLoader(loader_root_dir),
             keep_trailing_newline=True)
         jinja_env.globals.update(style_generator.GetGlobals())
         jinja_env.filters.update(style_generator.GetFilters())
-        template = jinja_env.get_template(path_to_template)
+        template = jinja_env.get_template(
+            path_overrides.GetPathToTemplate(path_to_template))
         return template.render(params)
 
     def Validate(self):

tools/style_variable_generator/path_overrides.py

+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import os
+
+
+# Returns the FileSystemLoader root directory. Overridden in google3.
+def GetFileSystemLoaderRootDirectory():
+    return os.path.dirname(os.path.realpath(__file__))
+
+
+# Returns the path to the template relative to FileSystemLoader root.
+# Overridden in google3.
+def GetPathToTemplate(path_to_template):
+    return path_to_template