Move ZoomController out of ChromeWebViewGuestDelegate.

This CL moves the ZoomController code for WebView out of
ChromeWebViewGuestDelegate and into GuestViewBase and WebViewGuest. This
will allow non-chrome builds to access the WebView zoom api.

BUG=none

Review URL: https://codereview.chromium.org/787813004

Cr-Commit-Position: refs/heads/master@{#308094}
10 files changed