Update fetch / loader classes to use OVERRIDE / FINAL when needed

Update fetch / loader classes to use OVERRIDE / FINAL when needed. Those are
useful for error checking or performance when the compiler supports it.

R=esprehn, abarth
BUG=333316

Review URL: https://codereview.chromium.org/134443002

git-svn-id: svn://svn.chromium.org/blink/trunk@164935 bbb929c8-8fbe-4397-9dbb-9b2b20218538