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Currently, there is only one instance of a PluginController at a time,
but references to it only live inside PDFViewerBaseElement and derived
classes. Therefore, descendants of the <pdf-viewer> element fire an
event handled by the PDFViewerElement. Making the controller a
singleton allows other elements to directly access the controller using
PluginController.getInstance().

In making PluginController a singleton, the body of the constructor is
moved to an init() method, because getInstance() creates a new object
using the default constructor. Consumers of the PluginController would
need to make sure the singleton has been initialized before using it.

Eventually, InkController will also be altered to be a singleton,
similarly to PluginController.

Bug: 1134208
Change-Id: I5f8a6b565e6fd565e67c41d841ab92fec43e46c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495504Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820941}