Add is_coalesced to latency info trace event

Currently, when recording in devtool performance tab, it will draw all
events even though some GestureScrollUpdate are coalesced into others.
In that case, coalesced GSUs are terminated earlier, and the slice are
smaller than what it should be and meaningless (as it's not really been
handled)

This CL adds a is_coalesced bit to latencyInfo trace event data, it will
be used for filtering out unnecessary GSUs in devtool performance
panel(crrev.com/c/1928916).

Bug: 956681
Change-Id: I9d999dd86e54f2b75d0a2527a4ba0e39f0e6ce9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1924311Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717657}

ui/latency/latency_info.cc

@@ -333,6 +333,7 @@ LatencyInfo::AsTraceableData() {
     record_data->Set(GetComponentName(lc.first), std::move(component_info));
   }
   record_data->SetDouble("trace_id", static_cast<double>(trace_id_));
+  record_data->SetBoolean("is_coalesced", coalesced_);
   return LatencyInfoTracedValue::FromValue(std::move(record_data));
 }