This is a step towards removing all instances of implicit any types
from turbolizer.

This CL also replaces var with const/let. This improves readability
and warnings.

Change-Id: I67c2974df209f857e67dfdbb743ce695ce861982
Notry: true
Bug: v8:7327
Reviewed-on: https://chromium-review.googlesource.com/c/1396419
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58572}