tools/gn: depend on allocator_extension.cc to keep bootstrap working

This patch adds the missing base/allocator/allocator_extension.cc to
the sources list to keep the boostrap process working and to fix the
undefined reference error as below:

malloc_dump_provider.cc:(.text+0x92): undefined reference to
`base::allocator::GetNumericProperty(char const*, unsigned long*)'

BUG=None
TEST=./tools/gn/bootstrap/bootstrap.py
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1511203002

Cr-Original-Commit-Position: refs/heads/master@{#364053}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 22d5facb2fef65c3085477319a96eeadf3650d3f

tools/gn/bootstrap/bootstrap.py

diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index 56567aa..8581f71 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -157,6 +157,7 @@
       'base/third_party/superfasthash/superfasthash.c',
   ])
   static_libraries['base']['sources'].extend([
+      'base/allocator/allocator_extension.cc',
       'base/at_exit.cc',
       'base/base_paths.cc',
       'base/base_switches.cc',