Rename Distance.* file to DistanceEffect.*

The class is named DistanceEffect, but the implementation is in the
files Distance.{h,cpp}.  Rename the file to match the class name.

No other changes.

BUG=718967
TEST=no changes to tests

Review-Url: https://codereview.chromium.org/2864383002
Cr-Commit-Position: refs/heads/master@{#470401}
4 files changed