Move down memory metric trigger for utility processes

This is moving the threshold to report a slow-reports with memory dump
for a utility process. The chosen value is 250M, which is the 98th
percentile based on UMA metrics.

We are expecting to get about 3x more reports. The amount of reports is
currently really low and harder to be actionnable. We believe this will
help find new potential memory leaks in utility processes.

R=erikchen@chromium.org

Change-Id: I93f286d4603d612b4ad64f1573d5967a6ba87d0b
Reviewed-on: https://chromium-review.googlesource.com/c/1315647Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604995}

chrome/browser/profiling_host/background_profiling_triggers.cc

@@ -44,7 +44,7 @@ const int kControlPopulationSamplingRate = 100;
 const size_t kBrowserProcessMallocTriggerKb = 400 * 1024;    // 400 MB
 const size_t kGPUProcessMallocTriggerKb = 400 * 1024;        // 400 MB
 const size_t kRendererProcessMallocTriggerKb = 500 * 1024;   // 500 MB
-const size_t kUtilityProcessMallocTriggerKb = 400 * 1024;    // 400 MB
+const size_t kUtilityProcessMallocTriggerKb = 250 * 1024;    // 250 MB
 
 // If memory usage has increased by 500MB since the last report, send another.
 const uint32_t kHighWaterMarkThresholdKb = 500 * 1024;  // 500 MB