Make WebView work with either enteredViewCallback or attachedCallback.

The Custom Elements enteredViewCallback is being renamed to attachedCallback.
This sets up both properties for WebView. After the Blink-side renaming, the
enteredViewCallback property can be removed.

BUG=180965
TEST=WebViewTest.*,WebViewInteractiveTest.*
R=fsamuel@chromium.org

Review URL: https://codereview.chromium.org/119743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242023 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242023