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}
const base::Feature* FindFeatureExposedToJava(const std::string& feature_name) {
  for (size_t i = 0; i < base::size(kFeaturesExposedToJava); ++i) {
    if (kFeaturesExposedToJava[i]->name == feature_name)
      return kFeaturesExposedToJava[i];
  }
  NOTREACHED() << "Queried feature cannot be found in ChromeFeatureList: "
               << feature_name;
  return nullptr;
}

const base::Feature* FindFeatureExposedToJava(const std::string& feature_name) {
  for (size_t i = 0; i < base::size(kFeaturesExposedToJava); ++i) {
    if (kFeaturesExposedToJava[i]->name == feature_name)
      return kFeaturesExposedToJava[i];
  }
  NOTREACHED() << "Queried feature cannot be found in ChromeFeatureList: "
               << feature_name;
  return nullptr;
}
