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In previous CLs, we've been working up from the bottom of the widget hierarchy
to rewrite Garden-O-Matic in Polymer. This CL moves this project forward by
starting from the top of the widget hierarchy. When these two efforts meet in
the middle, we'll be done!

This CL is mostly about fleshing out the structure of the components. We'll
need to iterate on the visual style of each component in future CLs. I've added
dependencies on Polymer's core and paper elements because the core elements are
super useful and the paper elements are super pretty.

The top-down version is hosted in gom2.html to avoid breaking the working
product. Currently, gom2 shows correctly grouped sets of failing tests and the
regression ranges for each group (albeit without any fancy styling).

NOTRY=true

Review URL: https://codereview.chromium.org/345853005

git-svn-id: svn://svn.chromium.org/blink/trunk@177172 bbb929c8-8fbe-4397-9dbb-9b2b20218538