Revert "Decouple the Custom Elements web platform feature and embedder feature."

This reverts r156331, which caused compile failures on multiple platforms
(Mac, Android, ChromeOS at least), see, e.g.,
http://build.chromium.org/p/chromium.webkit/builders/Mac10.8%20Tests/builds/2789/steps/compile/logs/stdio

>    Decouple the Custom Elements web platform feature and embedder feature.
>
>    Chrome Apps V2 implements <webview> as a Custom Element by flipping
>    on the Custom Elements feature, but in a limited mode where it only
>    accepts specific tag names. This breaks author script which tries to
>    do feature detection of Custom Elements, because document.register is
>    present, but acting in lame-duck mode.
>
>    This patch adds a specific API for embedders to define Custom
>    Elements. The API surface is independent of the web platform feature
>    so that they can be used in isolation or both at the same time.
>
>    BUG=273126
>
>    Review URL: https://chromiumcodereview.appspot.com/22854017

TBR=dominicc@chromium.org
BUG=273126

Review URL: https://codereview.chromium.org/22914015

git-svn-id: svn://svn.chromium.org/blink/trunk@156336 bbb929c8-8fbe-4397-9dbb-9b2b20218538