Minor cleanups in plugins_ui.cc.
Minor cleanups in plugins_ui.cc.

Specifically breaking down PluginsDOMHandler::PluginsLoaded method because
it is about 170 lines long. This is in preparation of Mojo-ifying
chrome://plugins.

BUG=570143

Review URL: https://codereview.chromium.org/1548573002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367645}

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