Add captured sites test directory to gitignore.

This prevents git clean -dff from removing the captured site test files cpid downloads at the start of the 'bot update' job.

Bug: 885350
Change-Id: I95b5a79fe6c5189f7d641ae936ca640761767f71
Reviewed-on: https://chromium-review.googlesource.com/c/1475152
Auto-Submit: Yiming Zhou <uwyiming@google.com>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632700}

.gitignore

@@ -131,6 +131,7 @@ vs-chromium-project.txt
 /chrome/test/chromeos/autotest/files/client/deps/perf_data_dep/test_src/
 /chrome/test/chromeos/autotest/files/client/deps/pyauto_dep/test_src/
 /chrome/test/chromeos/autotest/files/client/deps/telemetry_dep/test_src/
+/chrome/test/data/autofill/captured_sites
 /chrome/test/data/extensions/api_test/permissions/nacl_enabled/bin
 /chrome/test/data/firefox2_profile/searchplugins
 /chrome/test/data/firefox2_searchplugins