Move VariationsParamAssociator to base

Moves VariationsParamAssociator to base as FieldTrialParamAssociator
and just reroutes the calls in variations_associated_data.cc to this
new class in field_trial_param_associator.h/.cc. This is so we can eventually get field trial parameters into child processes using shared memory.

BUG=660038

Review-Url: https://codereview.chromium.org/2456723004
Cr-Commit-Position: refs/heads/master@{#428710}
4 files changed