Allow chromium buildtools path to be overriden in the environment.

gclient_utils.py

Index: gclient_utils.py
diff --git a/gclient_utils.py b/gclient_utils.py
index f0d8cec6f49cb99637af1c11ac8e5e54d5ef8bb1..e8d916e78d5d2a8aeba4cbf61bebf7b64ad79084 100644
--- a/gclient_utils.py
+++ b/gclient_utils.py
@@ -658,6 +658,13 @@ def GetMacWinOrLinux():
 def GetBuildtoolsPath():
   """Returns the full path to the buildtools directory.
   This is based on the root of the checkout containing the current directory."""
+
+  # Overriding the build tools path by environment is highly unsupported and may
+  # break without warning.  Do not rely on this for anything important.
+  override = os.environ.get('CHROMIUM_BUILDTOOLS_PATH')
+  if override is not None:
+    return override
+
   gclient_root = FindGclientRoot(os.getcwd())
   if not gclient_root:
     # Some projects might not use .gclient. Try to see whether we're in a git