perflib: Fix overflow error during zooming.

To zoom at a location, we need to first translate that location
to the origin, apply the zoom and translate back. The translation
was done using integers, and that causes precision errors with
longer traces. This CL fixes the issue by using doubles instead
of ints.

Change-Id: I33286ef66a6928726af4e95b4cfb433b6f7a1a44
1 file changed