Scrolling web view shows/hides chrome.

Whitespace tap only reveals chrome, never hides it.

Title bar tap to scroll to top reveals menu as well.

One pixel line at bottom of nav bar now appears if nav
is scrolled away.

Change-Id: I39b41b8e7bb34fbef540cbbbf0cd9cbb8703c2b8
Merge "Scrolling web view shows/hides chrome."