[typescript] TypeScript-ify timeline/CountersGraph.js
[typescript] TypeScript-ify timeline/CountersGraph.js

Bug: 1011811
Change-Id: Ic939801cc06778710ba65ec2809e48350c4f2efe
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2461770
Commit-Queue: Peter Marshall <6d0eecbeb7fb9d172f49b4b32481d2466fd89429@chromium.org>
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

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