Change WebTimeRange to use doubles instead of floats.

All the Chromium-side code that creates WebTimeRanges uses doubles and all
the Blink-side code that consumes WebTimeRanges use doubles, but somehow
this class didn't get updated. This change fixes the problem so that values
don't get truncated to the float range as they pass through a WebTimeRange.

Review URL: https://codereview.chromium.org/337923006

git-svn-id: svn://svn.chromium.org/blink/trunk@176664 bbb929c8-8fbe-4397-9dbb-9b2b20218538