Add a trace event for memory purging.

This CL is adding a trace event to detect when the browser is
triggering memory purging.

The memory purging is causing caches to be purge into the renderers.
As an example, the FontCache may be purged.

We are seeing slow-reports that demonstatrate slow cases related to
font cache management. This trace event will help figuring out if the
purging feature is the cause of these cases.

R=kenrb@chromium.org

Bug: 867931
Change-Id: I73061160637001b4b457c6319ead07d818ae5c7a
Reviewed-on: https://chromium-review.googlesource.com/c/1453057Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629181}

content/browser/renderer_host/render_process_host_impl.cc

@@ -2531,6 +2531,7 @@ bool RenderProcessHostImpl::IsKeepAliveRefCountDisabled() {
 }
 
 void RenderProcessHostImpl::PurgeAndSuspend() {
+  TRACE_EVENT0("renderer_host", "RenderProcessHostImpl::PurgeAndSuspend");
   GetRendererInterface()->ProcessPurgeAndSuspend();
 }