gyp_chromium: Better parsing of -G command line flag.

Handle -G output_dir=foo as well as -Goutput_dir=foo.
I'm not sure how common this case is but we have
NaCl SDK bot that do this.

Getting this wrong results in windows toolchain being
installed in 'out/' (which doesn't necessarily exist)
rather than the output_dir tree and gyp uses.

Add unittests for the affected parts of gyp_chromium.

BUG=454594

Review URL: https://codereview.chromium.org/896663002

Cr-Commit-Position: refs/heads/master@{#314381}
diff --git a/build/PRESUBMIT.py b/build/PRESUBMIT.py
new file mode 100644
index 0000000..3c70193
--- /dev/null
+++ b/build/PRESUBMIT.py
@@ -0,0 +1,18 @@
+# Copyright 2015 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.
+
+WHITELIST = [ r'.+_test.py$' ]
+
+
+def _RunTests(input_api, output_api):
+  return (input_api.canned_checks.RunUnitTestsInDirectory(
+          input_api, output_api, '.', whitelist=[r'.+_test.py$']))
+
+
+def CheckChangeOnUpload(input_api, output_api):
+  return _RunTests(input_api, output_api)
+
+
+def CheckChangeOnCommit(input_api, output_api):
+  return _RunTests(input_api, output_api)