metrics: Clear the global memory dump once data has been logged.

The global memory dump is fetched once per memory metrics recording, and
never re-used. However the global dump is never reset, wasting memory.

Bug: 1054360
Change-Id: I04f65427194068e579cd87938d4cf65af9bd0962
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056820
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743056}

chrome/browser/metrics/process_memory_metrics_emitter.cc

@@ -757,8 +757,7 @@ void ProcessMemoryMetricsEmitter::CollateResults() {
 
   if (memory_dump_in_progress_ || get_process_urls_in_progress_)
     return;
-  if (!global_dump_)
-    return;
+  DCHECK(global_dump_);
 
   uint32_t private_footprint_total_kb = 0;
   uint32_t renderer_private_footprint_total_kb = 0;
@@ -909,6 +908,8 @@ void ProcessMemoryMetricsEmitter::CollateResults() {
     // processes.
     per_tab_metrics.RecordPmfs(GetUkmRecorder());
   }
+
+  global_dump_ = nullptr;
 }
 
 namespace {