Added new "backend" flag to lottie-web-perf.js and created a new lottie-web-canvas-perf.html file.
Bug: skia:9237
Change-Id: Ibca2f121666daf25fbbccac48fed80eb12ccedf7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229420
Commit-Queue: Ravi Mistry <rmistry@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
