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 GetURLFromValue(const base::Value* in_value, std::string* out_value) {
  return in_value && out_value && in_value->GetAsString(out_value);
}

bool GetURLFromValue(const base::Value* in_value, std::string* out_value) {
  return in_value && out_value && in_value->GetAsString(out_value);
}
