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}
VariationsRequestSchedulerMobile::VariationsRequestSchedulerMobile(
    const base::Closure& task,
    PrefService* local_state) :
  VariationsRequestScheduler(task), local_state_(local_state) {
}

VariationsRequestSchedulerMobile::VariationsRequestSchedulerMobile(
    const base::Closure& task,
    PrefService* local_state) :
  VariationsRequestScheduler(task), local_state_(local_state) {
}
