Make ScrollOffsetAnimationCurve::UpdateTarget use TimeDelta

Makes the method use TimeDelta instead of double to represent
duration.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Id95aface04fa2b72bc919fd4ab52af062908f8df
Bug: 763980
Reviewed-on: https://chromium-review.googlesource.com/1140355
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577928}
7 files changed