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

Bug: 1011811
Change-Id: Ib532ea492ab36b5edbb85bcbf77c705687132dcf
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2431518
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Commit-Queue: Peter Marshall <6d0eecbeb7fb9d172f49b4b32481d2466fd89429@chromium.org>

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