Revert "Move GN enable_plugins variable out of //ppapi"

This reverts commit 0234539437a5596ef2c5bc9d45629b1d66305ce2.

Reason for revert: linux-cast-arm-rel build failures https://cr-buildbucket.appspot.com/build/8709237076730407729

Bug: 40511450
Original change's description:
> 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}

Bug: 40511450
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I65088d72dab82d99d720ab4c68e823bb7bc389b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6756624
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Lei Zhang <thestig@chromium.org>
Auto-Submit: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1487268}
NOKEYCHECK=True
GitOrigin-RevId: 192a03ca8014335b19b92398bc414f671a923850