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In this patch, we make 3 changes to measure pinch zoom latency:
1. Separate touchpad pinch zoom latency info from wheel
2. Add histogram to measure pinch zoom latency from when receiving
   event from system to when sending event to render. This helps us
   understand the latency of waiting synthetic wheel event.
3. Add histogram to measure pinch zoom latency from when receiving
   event from system to when the resulting GPU frame swap completed.
   This helps us understand the latency of the end to end touchpad
   pinch.


Bug: 565980
Change-Id: I304675684746e6d8f6001937264afa0fc9037324
Reviewed-on: https://chromium-review.googlesource.com/1142447Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579911}