<shadow> should not select UA shadow nodes if it's in the oldest author shadow and its host is not supportsShadowElementForUserAgentShadow, and enable author shadows for <details> and <summary> again.

Also, print a warning about this behavior when the <shadow> is attached
to a document.

Restore tests removed in https://src.chromium.org/viewvc/blink?view=rev&revision=155391


BUG=234020,253801

Review URL: https://chromiumcodereview.appspot.com/21115003

git-svn-id: svn://svn.chromium.org/blink/trunk@155577 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Measure the usage of ::-webkit-distributed() CSS pseudo element.

::-webkit-distributed() CSS pseudo element is part of Shadow DOM specification.
We are in process of unprefixing Shadow DOM-related APIs, and would like to
understand the usage of the prefixed APIs.

A (non-static) member function count() is added to UseCounter because it is
hard to obtain a Document or DOMWindow within CSSParser and a pointer to
UseConter is already available in CSSParser.

BUG=242153
R=dglazkov@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21713002

git-svn-id: svn://svn.chromium.org/blink/trunk@155578 bbb929c8-8fbe-4397-9dbb-9b2b20218538