<webview>: Support zoom in WebUI

This CL plumbs the embedder's zoom into the guest WebContents.

BUG=330818

Review URL: https://codereview.chromium.org/98073015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244116 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244116