Re-land: base: Introduce an explicit call for reducing emulated discardable memory usage.

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index e652d229f3a8dafb4e6b58e018370886ba4de931..a438f5cef23aa991bbb3ca2fa6dc619cdaf3763f 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -15,6 +15,7 @@
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/discardable_memory.h"
+#include "base/memory/discardable_memory_emulated.h"
 #include "base/memory/shared_memory.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
@@ -534,10 +535,6 @@ void RenderThreadImpl::Init() {
 
   base::DiscardableMemory::SetPreferredType(type);
 
-  // Allow discardable memory implementations to register memory pressure
-  // listeners.
-  base::DiscardableMemory::RegisterMemoryPressureListeners();
-
   // AllocateGpuMemoryBuffer must be used exclusively on one thread but
   // it doesn't have to be the same thread RenderThreadImpl is created on.
   allocate_gpu_memory_buffer_thread_checker_.DetachFromThread();
@@ -1609,6 +1606,11 @@ void RenderThreadImpl::WidgetHidden() {
   hidden_widget_count_++;
 
   if (widget_count_ && hidden_widget_count_ == widget_count_) {
+    // We emulate a "moderate memory pressure" event for emulated discardable
+    // memory to make sure that background tabs don't use too much memory.
+    base::DiscardableMemoryEmulated::
+        ReduceMemoryUsageUntilWithinModeratePressureLimit();
+
     if (GetContentClient()->renderer()->RunIdleHandlerWhenWidgetsHidden())
       ScheduleIdleHandler(kInitialIdleHandlerDelayMs);
   }