In TimeStamp_windows.cpp and TimeStamp_darwin.cpp, in
TimeStamp::FromMilliseconds we cast the floating-point number of ticks to
a 64-bit integer before passing to TimeStamp::FromTicks(int64_t).
This means that we skip the check for integer overflow performed by
TimeStamp::FromTicks(double).
This patch simply removes that cast so that we perform overflow checking.
It also adds an assertion to ElementAnimation since this is one place where
the lack of overflow checking was producing a negative value where it should
not.
