Add public API for on screen zoom controls

    http://b/issue?id=3378224
    This is a simple unhide.
    Allow the use of WebView pinch-to-zoom gestures
    without the on screen zoom controls.
    Currently, enabling pinch to zoom with the public API also shows the
    on screen zoom buttons.
    If the zoom buttons are not wanted, a custom implementation of pinch to
    zoom is required, which can't be done correctly at this point.

Change-Id: I3f94080471c3a4b498b11c91603fff9994dc3c2c