Scrollable should settle back to 0.0

Scrollable should settle back to 0.0

This CL teaches Scrollable how to settle back to a scroll offset of 0.0 after a
fling or a scroll. There's still some room for improvement:

1) Some of this logic should be factored out into the scroll curve object.
2) We don't produce the correct animation curves when we fling into the
   overscroll region because we wait for the fling velocity to reach zero
   before we start the settling animation.

R=ojan@chromium.org, eseidel@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/ce1c56c8c532bae98e9b9ffb71e3a98d8cae1fe3

[ojan, 2015-03-12 21:32:40]

ojan@chromium.org changed reviewers:
+ ojan@chromium.org

[ojan, 2015-03-12 21:32:40]

lgtm

[abarth-chromium, 2015-03-12 21:33:40]

Committed patchset #1 (id:1) manually as
ce1c56c8c532bae98e9b9ffb71e3a98d8cae1fe3 (presubmit successful).