latency: Remove an unused LatencyInfo param.

The |event_count| param is no longer used for anything. So
remove this, and update the usage in the code.

BUG=731202
TBR=tdresser@ as the API owner, for mechanical changes in cc/

Change-Id: Ifd6428fe74086ec621e81d8acec8b21db573cae8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1691084
Commit-Queue: Sadrul Chowdhury <[email protected]>
Reviewed-by: Timothy Dresser <[email protected]>
Cr-Commit-Position: refs/heads/master@{#675397}
20 files changed