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Expanding or collapsing a section takes 350ms, but fading the new content
in and the old content out takes 500ms. This is a contributing factor to
the overlap seen when closing a section, as the old section is still
visible when the collapse animation has completed.

Shortening the fade duration to match the expand/collapse animation makes
that overlap much less likely to occur. It also makes the animations
appear snappier, IMO -- if the page is still fading after expanding, it
looks slugging.

This re-implements the fade animations, rather than programmatically
changing the neon-animated-pages' transition durations for the
existing fade-in-animation/fade-out-animation. The latter is not possible
due to https://github.com/PolymerElements/neon-animation/issues/209.

R=dbeam@chromium.org
BUG=716680
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2856223007
Cr-Commit-Position: refs/heads/master@{#469582}