Set default values for timeout and maximumAge of PositionOptions

Geolocation spec defines default values for both timeout and 
maximumAge but current implementation is working with
hasTimeout and hasMaximumAge functions for emulating that.

Setting initial values of timeout and maximumAge is more proper than using these "has" functions.
In addition, hasZeroTimeout can be removed because it is
enough to check timeout is zero or not and clearMaximumAge
can also be removed, as it is enough to set 0 to maximumAge.

BUG=368184

Review URL: https://codereview.chromium.org/297143003

git-svn-id: svn://svn.chromium.org/blink/trunk@175131 bbb929c8-8fbe-4397-9dbb-9b2b20218538