Add observer_list_threadsafe.cc to gn bootstrap deps

After 24423de1dc711cebf73d097943d2090ce4d8aed7 [0] gn boostrap fails
with

  undefined reference to `base::internal::ObserverListThreadSafeBase::tls_current_notification_'

Add observer_list_threadsafe.cc that contains that static var to
bootstrap.py to make it work again.

[0] https://chromium-review.googlesource.com/740989

Bug: 783188
Change-Id: Iac583c52b8aed452b3bdd5b33a60857e2e8285b9
Reviewed-on: https://chromium-review.googlesource.com/758584
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#515978}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 976fe5f366eb4c6bf1af6c1d9c871ba7546cd5bb

tools/gn/bootstrap/bootstrap.py

diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index 8300c3b..7098995 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -482,6 +482,7 @@
       'base/metrics/sample_vector.cc',
       'base/metrics/sparse_histogram.cc',
       'base/metrics/statistics_recorder.cc',
+      'base/observer_list_threadsafe.cc',
       'base/path_service.cc',
       'base/pending_task.cc',
       'base/pickle.cc',