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}
int ChromeBrowserMainParts::PreCreateThreads() {
  TRACE_EVENT0("startup", "ChromeBrowserMainParts::PreCreateThreads");
  result_code_ = PreCreateThreadsImpl();

  if (result_code_ == content::RESULT_CODE_NORMAL_EXIT) {
#if !defined(OS_ANDROID)
    DCHECK(master_prefs_.get());
    DCHECK(browser_creator_.get());
#endif  // !defined(OS_ANDROID)
    for (size_t i = 0; i < chrome_extra_parts_.size(); ++i)
      chrome_extra_parts_[i]->PreCreateThreads();
  }

  return result_code_;
}

int ChromeBrowserMainParts::PreCreateThreads() {
  TRACE_EVENT0("startup", "ChromeBrowserMainParts::PreCreateThreads");
  result_code_ = PreCreateThreadsImpl();

  if (result_code_ == content::RESULT_CODE_NORMAL_EXIT) {
#if !defined(OS_ANDROID)
    DCHECK(master_prefs_.get());
    DCHECK(browser_creator_.get());
#endif  // !defined(OS_ANDROID)
    for (size_t i = 0; i < chrome_extra_parts_.size(); ++i)
      chrome_extra_parts_[i]->PreCreateThreads();
  }

  return result_code_;
}
