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

This reverts commit d8e3222d.

Reason for revert: crbug.com/1055093

Original change's description:
> 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}

TBR=asvitkine@chromium.org,lizeb@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

TBR: asvitkine
Bug: 1054360, 1055093
Change-Id: I313c6c511cb422f69998f1c4d0f7d2ecf602da04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2069320
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743902}

chrome/browser/metrics/process_memory_metrics_emitter.cc

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