Better handle double tap for smooth reflow and zoom.

In case it's the first time to reflow or the page is in zoom overview
mode, let the reflow and zoom happen at the same time to avoid the
need of double tapping twice.

Also fix the viewport width to webview's original width for mobile
site.

issue: 4482888
Change-Id: Ic7c73a9eb3f75fd68322f7f4da14141126e75ad8