Upgrade build and clang.

The issue with //testing/multiprocess_func_list has been fixed in upstream
//build. This Cl upgrades //build to ToT.

In order to do so clang must also be updated. The Wno-undefined-var-template
is only passed if the pinned clang revision isn't 266460, which is the
currently pinned clang revision in chromium. This is there so that the bot
that builds with trunk clang gets this flag, but pinned clang doesn't.

We accidentally pickup the flag since our pinned clang is different from the
Chrome version. So, we must always roll //build and clang together when
changing deps or bad things may happen as there is an implicit dependency
due to compile flags.

BUG=pdfium:106

Review URL: https://codereview.chromium.org/1911483002
diff --git a/build_overrides/gtest.gni b/build_overrides/gtest.gni
new file mode 100644
index 0000000..6089962
--- /dev/null
+++ b/build_overrides/gtest.gni
@@ -0,0 +1,9 @@
+# Copyright 2016 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.
+
+# Exclude support for registering main function in multi-process tests.
+gtest_include_multiprocess = false
+
+# Exclude support for platform-specific operations across unit tests.
+gtest_include_platform_test = false