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Along the way, clean it up a lot based on lessons learned from my first chromium code.

    Note this changes PpdReference to use just a single "effective_make_and_model"
    field to resolve ppds from the ppd server.  This closely mirrors what's usually available from IPP and other sources, so seems to be the most sensible key for doing ppd lookups.

    This also updates all callsites and cascading changes to make the provider changes work.

BUG=617253
R=skau
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2613683004
Cr-Commit-Position: refs/heads/master@{#448100}