Add feature and flag to enable incognito Chrome Custom Tabs

kCCTIncognito feature and flag are added to enable/disable incognito
Chrome Custom Tabs. The default is set to disabled.

Bug: 1023759
Change-Id: If32d256e3e9eaa94bcc09f7538c85e2dab53c589
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911201
Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714849}
std::vector<FeatureEntry>* GetEntriesForTesting() {
  static base::NoDestructor<std::vector<FeatureEntry>> entries;
  return entries.get();
}

std::vector<FeatureEntry>* GetEntriesForTesting() {
  static base::NoDestructor<std::vector<FeatureEntry>> entries;
  return entries.get();
}
