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Makes the collapsing card position: absolute (instead of fixed).  Anchoring
it to the section makes it actually follow the section, so it always moves
toward the right position even if the containter's scroll/height/size changes.
Should reduce jarring jumps at the end of transitions.

Also moves the style changes into Web Animations instead of inline. This makes
the animations fire-and-forget. Transition clean up is largely automatic, there
are no styles to (forget to) remove (at the wrong time). If a transition is
buggy, we're still likely to end up in the right place.

This fixes several animation bugs (but likely introduces new ones).

BUG=589681, 621245, 622172
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2230123002
Cr-Commit-Position: refs/heads/master@{#414289}