It took me a while to understand that the {index} is actually a
signature index. This CL changes the name to {sig_index} to make this
more clear.

Drive-by: Fix a CHECK to check the canonical signature index instead of
the original index. This ensures that there is a canonical signature
index in the signature map.
Drive-by^2: Un-templatize a method.

R=titzer@chromium.org

Change-Id: Ifdaec59806c4d5c976170807596503d2874f04e4
Reviewed-on: https://chromium-review.googlesource.com/871190Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50695}