Move variations prefs into the variations component

These prefs are used by variations code that is targeted for componentization.

BUG=382865
TBR=thakis

Review URL: https://codereview.chromium.org/1265423003

Cr-Commit-Position: refs/heads/master@{#343661}
bool MasterPreferences::GetExtensionsBlock(
    base::DictionaryValue** extensions) const {
  return master_dictionary_->GetDictionary(
      master_preferences::kExtensionsBlock, extensions);
 }

bool MasterPreferences::GetExtensionsBlock(
    base::DictionaryValue** extensions) const {
  return master_dictionary_->GetDictionary(
      master_preferences::kExtensionsBlock, extensions);
 }
