[utilities] Replace Array with Set for pending animations
[utilities] Replace Array with Set for pending animations

This CL turns AnimationModel._pendingAnimations from an Array into a
Set as it is used as such. This is in preparation for the removal of
Array#remove from utilities.js.

Bug: chromium:1050549
Change-Id: Ic5419e81e8edd2a5aa6fb03dad7cfa8a870a3545
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2080374
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend