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This removes a significant amount of code from the trusted plugin and
simplifies the callbacks that are used in PluginReverseInterface.

I was confused at first by the existing use of url_file_info_map_; it looks
like it was intended to serve as a cache for retrieved files, but I think the
files would be reopened every time, even if the file was already open. For
simplicity, I didn't try to add any caching behavior in this change.

Note that this changes progress events to be rate limited at 10ms per-file,
not 10ms per-plugin. It's a bit simpler to write this way, but I can
change that limit to be per-plugin if that makes more sense.

BUG=370556
R=bbudge@chromium.org

Review URL: https://codereview.chromium.org/294633003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272550 0039d316-1c4b-4281-b951-d872f2087c98