The GC time for a task is defined by the sum of GC duration
overlapping with the task's duration.

Also, grey out non-overlapping slices in the task-oriented
trace view.

R=go1.11

Change-Id: I42def0eb520f5d9bd07edd265e558706f6fab552
Reviewed-on: https://go-review.googlesource.com/90219Reviewed-by: Heschi Kreinick <heschi@google.com>