Use proper ToLength() operation in %ArrayConcat()

LOG=N
R=dslomov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/799853003

Cr-Commit-Position: refs/heads/master@{#25828}
Add infrastructure to keep track of references to prototypes.

There are no users of this infrastructure yet, so it's behind an off-by-default flag.

Review URL: https://codereview.chromium.org/768633002

Cr-Commit-Position: refs/heads/master@{#25829}