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This change updates the GetAllGrantedObjects() method of
UsbChooserContext to include the USB objects granted by the
WebUsbAllowDevicesForUrls policy. The GetAllGrantedObjects() method
removes user granted objects that are also granted permission by the
policy.

SiteSettingsHelper was modified to skip the policy objects until the UI
is ready to display them.

Design Doc:
https://docs.google.com/document/d/1MPvsrWiVD_jAC8ELyk8njFpy6j1thfVU5aWT3TCWE8w

Bug: 854329
Change-Id: I8442f588b0d7029062c67bbb2896cefeb558a887
Reviewed-on: https://chromium-review.googlesource.com/c/1297301
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609479}