Add missing source to gn bootstrap.

This command is subject to bitrot, and this seems to be today's
instance.

BUG=510240
TEST=tools/gn/bootstrap/bootstrap.py

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1235233005

Cr-Original-Commit-Position: refs/heads/master@{#338766}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d673f55c138f6b263375c096c43ecbb929246021

tools/gn/bootstrap/bootstrap.py

diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index 0cee76e..b1924d5 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -210,6 +210,7 @@
       'base/sequence_checker_impl.cc',
       'base/sequenced_task_runner.cc',
       'base/sha1_portable.cc',
+      'base/strings/pattern.cc',
       'base/strings/string16.cc',
       'base/strings/string_number_conversions.cc',
       'base/strings/string_piece.cc',