Enable zoom controls in HTMLViewer

Due to changes in Chromium WebView, text files may be displayed
using a smaller font, not comfortable for reading. Enabling zoom
controls serves as a workaround. This also puts HTMLViewer
on par with DocViewer (for office files), which also allows
zooming of contents.

BUG: 10674904
Change-Id: I806fbed103e6f714824f69d78e681ee96f142d42