Animations: Scale timeline when moving slider with keyboard
Animations: Scale timeline when moving slider with keyboard

This fixes a bug where the Animations drawer timeline doesn't adjust
its scale when the keyboard is used to lengthen an animation (but does
when the mouse is used). I broke the resizing logic from the mouse
handler into its own method so that the keyboard handler can take
advantage of it as well.

Video: https://i.imgur.com/ZEImHNN.mp4

Bug: 1042102
Change-Id: I571e400106a00d334a5f0becf25898d6af9878da
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2001815
Reviewed-by: Fabio Rocha <38c66cf695d5b4e90d3d1a3b74a5cd812c01b72d@microsoft.com>
Reviewed-by: Brian Cui <eebe54f29fdfbfde69930c5ed5d840454db62ce7@microsoft.com>
Reviewed-by: Brandon Goddard <37b103671c22e8d219b41940d66d3220ab618ae9@microsoft.com>
Reviewed-by: Jose Leal <49360abdd51ad00da751c3d448c2dbea47853f27@microsoft.com>
Commit-Queue: Jack Lynch <d6fb7cfbb8c8618e756350219cb7952a0ad656b7@microsoft.com>

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