Transfer date/time value to the chooser as a double

We want to avoid passing strings in IPCs. We also don't want string to date conversions in multiple places. Sending all types of values as double simplifies the code, since we already do it for min/max.

Adding WebDateTimeChooserParams::doubleValue.
Adding didChooseValue(double value)

BUG=None

Review URL: https://codereview.chromium.org/62083004

git-svn-id: svn://svn.chromium.org/blink/trunk@162480 bbb929c8-8fbe-4397-9dbb-9b2b20218538