Flag webviewTag API as internal

This prevents a bad namespace exposure to script contexts.

Also add dependencies to devtools features to suppress error messages.

BUG=304136
NOTRY=true

Review URL: https://codereview.chromium.org/25557003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227054 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227054