[ppapi] Prep moving ENABLE_PLUGINS into content/public/common

This will allow us to remove a bunch of #include ppapi inside
content without breaking the build everywhere.

Bug: 40511450
Change-Id: I1386dbcada5c4589c55af408bf2995080f6aa633
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6685852
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Auto-Submit: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Derek Schuff <dschuff@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1480158}
NOKEYCHECK=True
GitOrigin-RevId: 9cf63231e10aa5abc4479f063b33545bf986cfff