FeatureProvider returns std::map instead of vector

To reduce binding time as much as possible, use std::map(O(1)) instead of std::vector(O(n)).

Also, use 'const_iterator' to fix improper usage of 'auto'.

BUG=592537
TEST=Every tests are green.

Review URL: https://codereview.chromium.org/1772683002

Cr-Commit-Position: refs/heads/master@{#379711}
6 files changed