autotestPrivate.setWhitelistedPref supports pluginvm permission prefs

Bug: b/179216610
Change-Id: Ib394f24157bd988342fc892181712f0418f0635f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2670666


Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Patrick Meiring <meiring@google.com>
Commit-Queue: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#850443}