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Currently an event is fired to the top-level PDFViewerElement to make
requests to the plugin for painting thumbnails. Now that the
PluginController is a singleton accessible throughout the PDF Viewer's
UI code, make the plugin requests directly inside
ViewerThumbnailBarElement.

Bug: 1134208
Change-Id: I2867effddf43d06b0a6f8cf13fbe2e0881d0e0c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494245
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821110}