Remove the only tracing event in platform

platform/probe has only one tracing event: "PlatformResourceSendRequest"
which is called in ResourceFetcher::StartLoad. This is very similar to
what we are doing for "ResourceSendRequest" in
InspectorTraceEvent::WillSendRequest also called in
ResourceFetcher::StartLoad through
ResourceLoadObserver::WillSendRequest.

As the event has never been actually used, let's remove it.

This CL will allow us to remove PlatformProbeSink.

Bug: 914739
Change-Id: I0f93e363214ac6da071ee480ee155d3492fb3e7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616839
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662507}

third_party/blink/renderer/platform/probe/platform_trace_events_agent.cc

@@ -12,22 +12,12 @@ namespace blink {
 
 namespace {
 
-std::unique_ptr<TracedValue> BuildData(
-    const probe::PlatformSendRequest& probe) {
-  auto value = std::make_unique<TracedValue>();
-  value->SetString("id", String::Number(probe.identifier));
-  return value;
-}
-
 }  // namespace
 
 void PlatformTraceEventsAgent::Will(const probe::PlatformSendRequest& probe) {
-  TRACE_EVENT_BEGIN1("devtools.timeline", "PlatformResourceSendRequest", "data",
-                     BuildData(probe));
 }
 
 void PlatformTraceEventsAgent::Did(const probe::PlatformSendRequest& probe) {
-  TRACE_EVENT_END0("devtools.timeline", "PlatformResourceSendRequest");
 }
 
 }  // namespace blink