Bug 1698524: Clear profiler memory counter on re-enabling of the profiler r=gerald

Differential Revision: https://phabricator.services.mozilla.com/D108447

tools/profiler/core/memory_hooks.cpp

@@ -547,6 +547,8 @@ void install_memory_hooks() {
     // Also initialize the ThreadIntercept, even if native allocation tracking
     // won't be turned on. This way the TLS will be initialized.
     ThreadIntercept::Init();
+  } else {
+    sCounter->Clear();
   }
   jemalloc_replace_dynamic(replace_init);
 }

tools/profiler/public/ProfilerCounts.h

@@ -110,6 +110,14 @@ class BaseProfilerCount {
 #  endif
   }
 
+  void Clear() {
+    *mCounter = 0;
+    // We don't reset *mNumber or mPrevNumber.  We encode numbers as
+    // positive deltas, and currently we only care about the deltas (for
+    // e.g. heatmaps).  If we ever need to clear mNumber as well, we can an
+    // alternative method (Reset()) to do so.
+  }
+
   // We don't define ++ and Add() here, since the static defines directly
   // increment the atomic counters, and the subclasses implement ++ and
   // Add() directly.