Revert "Disable the new tab-loading animation"

This reverts commit 355b8185716292a8f145c77168ee89148ecabf31.

Reason for revert: Made several tests flaky: https://crbug.com/912543

Original change's description:
> Disable the new tab-loading animation
> 
> Makes sure that a lot of animation-related code is bypassed when the
> new-tab-animation flag is off. This should hopefully fix a couple of
> performance regressions that have not yet been root caused so that they
> don't go out with M72.
> 
> Bug: chromium:912328, chromium:905745, chromium:905918, chromium:910265
> Change-Id: Id3f131db427eb3ee1618d6c9683fd5e47dc134e8
> Reviewed-on: https://chromium-review.googlesource.com/c/1364212
> Reviewed-by: Sidney San Martín <sdy@chromium.org>
> Commit-Queue: Peter Boström <pbos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#614199}

TBR=pbos@chromium.org,sdy@chromium.org

Change-Id: Ib4c022a255ad085c1716d3559a7f84dcb61c2785
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:912328, chromium:905745, chromium:905918, chromium:910265
Reviewed-on: https://chromium-review.googlesource.com/c/1366359
Reviewed-by: Jeffrey Yasskin <jyasskin@chromium.org>
Commit-Queue: Jeffrey Yasskin <jyasskin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614440}
3 files changed