Deflake fast/dom/Geolocation/timestamp.html

On mock geolocation, the timestamp is actually set at the time of
calling setMockGeolocationPosition, which means that on a slower
machine, time may pass between this call and saving the current time
to check against. We prolly should fix that.

TBR=ojan
BUG=

Review URL: https://codereview.chromium.org/26450005

git-svn-id: svn://svn.chromium.org/blink/trunk@159153 bbb929c8-8fbe-4397-9dbb-9b2b20218538