Move GN enable_plugins variable out of //ppapi

Move the GN enable_plugins variable to //content/public/common, so it
lives in the same directory as the C++ ENABLE_PLUGINS buildflag.

Bug: 40511450
Change-Id: Ic73f19e510b5b5cc94ecda37519de764abd03fba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6757897
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Owners-Override: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1487258}
NOKEYCHECK=True
GitOrigin-RevId: 0234539437a5596ef2c5bc9d45629b1d66305ce2