Previously we reset runtime counters and dump them when we enter, exit top level
trace events respectively. However, there is gap between two top level trace
events and runtime counters may be activated, resetting the counters makes the
accumulated time inaccurate, and we may end up with negative time due to the
nature of how we accumulate time.

This patch fixes this problem by only resetting counters when there's no
counters active, and before dump counters, we traverse current active counters
to calculate their time, and then restart their timer.

BUG=chromium:658145

Review-Url: https://codereview.chromium.org/2457523002
Cr-Commit-Position: refs/heads/master@{#40653}