cc: Remove memory pressure handler from SoftwareImageDecodeCache

The memory pressure handler of SoftwareImageDecodeCache currently
purges all unlocked cache entries upon a critical memory pressure
notification. These cache entries are primarily composed of
discardable memory.

However, the DiscardableSharedMemoryManager purges all unlocked
discardable memory upon a critical memory pressure notification as well.
It is expected that all unlocked cache entries have their respective
discardable memory unlocked.

Remove the unnecessary memory pressure handler in order to:
- Reduce the number of memory pressure notification consumers
- Reduce work performed during memory pressure handling (cache walk)

Change-Id: I196b6da65ca961cdbc525869e419f37999d4f453
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951134Reviewed-by: Eric Karl <ericrk@chromium.org>
Auto-Submit: Gordon Guan <gordonguan@google.com>
Commit-Queue: Gordon Guan <gordonguan@google.com>
Cr-Commit-Position: refs/heads/master@{#722974}

cc/tiles/software_image_decode_cache.cc

@@ -169,9 +169,6 @@ SoftwareImageDecodeCache::SoftwareImageDecodeCache(
         this, "cc::SoftwareImageDecodeCache",
         base::ThreadTaskRunnerHandle::Get());
   }
-  memory_pressure_listener_.reset(new base::MemoryPressureListener(
-      base::BindRepeating(&SoftwareImageDecodeCache::OnMemoryPressure,
-                          base::Unretained(this))));
 }
 
 SoftwareImageDecodeCache::~SoftwareImageDecodeCache() {
@@ -684,19 +681,6 @@ bool SoftwareImageDecodeCache::OnMemoryDump(
   return true;
 }
 
-void SoftwareImageDecodeCache::OnMemoryPressure(
-    base::MemoryPressureListener::MemoryPressureLevel level) {
-  base::AutoLock lock(lock_);
-  switch (level) {
-    case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE:
-    case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE:
-      break;
-    case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL:
-      ReduceCacheUsageUntilWithinLimit(0);
-      break;
-  }
-}
-
 SoftwareImageDecodeCache::CacheEntry* SoftwareImageDecodeCache::AddCacheEntry(
     const CacheKey& key) {
   frame_key_to_image_keys_[key.frame_key()].push_back(key);

cc/tiles/software_image_decode_cache.h

@@ -12,7 +12,6 @@
 #include <vector>
 
 #include "base/containers/mru_cache.h"
-#include "base/memory/memory_pressure_listener.h"
 #include "base/memory/ref_counted.h"
 #include "base/numerics/safe_math.h"
 #include "base/thread_annotations.h"
@@ -112,9 +111,6 @@ class CC_EXPORT SoftwareImageDecodeCache
   void ReduceCacheUsageUntilWithinLimit(size_t limit)
       EXCLUSIVE_LOCKS_REQUIRED(lock_);
 
-  void OnMemoryPressure(base::MemoryPressureListener::MemoryPressureLevel level)
-      LOCKS_EXCLUDED(lock_);
-
   // Helper method to get the different tasks. Note that this should be used as
   // if it was public (ie, all of the locks need to be properly acquired).
   TaskResult GetTaskForImageAndRefInternal(const DrawImage& image,
@@ -142,9 +138,6 @@ class CC_EXPORT SoftwareImageDecodeCache
   // Decoded images and ref counts (predecode path).
   ImageMRUCache decoded_images_ GUARDED_BY(lock_);
 
-  std::unique_ptr<base::MemoryPressureListener> memory_pressure_listener_
-      GUARDED_BY(lock_);
-
   // A map of PaintImage::FrameKey to the ImageKeys for cached decodes of this
   // PaintImage.
   std::unordered_map<PaintImage::FrameKey,