Declare GetShadowRoot and IsInUserAgentShadowRoot as inline.

They are defined as inline in shadow_root.h, and I got surprised by some
linking errors when attempting to move code that called
IsInUserAgentShadowRoot(). Let's have compile errors instead!

Also move some inlined shadow root related functions from element.h into
shadow_root.h. They didn't cause any trouble as it was (since every
compilation unit actually calling them apparently were lucky enough to
include shadow_root.h), but would cause trouble now.

Change-Id: Ib4f356571dc737f59e0fca7751da1fece94d0592
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6243240
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1418026}