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Change the transforms of the windows in overview mode to better match the mocks.
Notable change in the code is that instead of computing the terminal transforms
at the start, and then computing the intermediate transforms from those, we
compute less amount of information at start, and compute the transform at each
state from those. This makes it easier to perform non-linear transforms during
the scrolls.

BUG=403849
R=oshima@chromium.org

Review URL: https://codereview.chromium.org/556043002

Cr-Commit-Position: refs/heads/master@{#294108}