Move purging from OnMemoryState to OnPurgeMemory() in cc/

cc/tiles/software_image_decode_cache.cc

Index: cc/tiles/software_image_decode_cache.cc
diff --git a/cc/tiles/software_image_decode_cache.cc b/cc/tiles/software_image_decode_cache.cc
index 53758c02552fe6e00642649291ca228f984df56b..13ed05129fc5ed5aa18d4b601f743689777f5e44 100644
--- a/cc/tiles/software_image_decode_cache.cc
+++ b/cc/tiles/software_image_decode_cache.cc
@@ -797,12 +797,10 @@ void SoftwareImageDecodeCache::UnrefAtRasterImage(const ImageKey& key) {
   }
 }
 
-void SoftwareImageDecodeCache::ReduceCacheUsage() {
+void SoftwareImageDecodeCache::ReduceCacheUsageUntilWithinLimit(size_t limit) {
   TRACE_EVENT0("cc", "SoftwareImageDecodeCache::ReduceCacheUsage");
-  base::AutoLock lock(lock_);
-  size_t num_to_remove = (decoded_images_.size() > max_items_in_cache_)
-                             ? (decoded_images_.size() - max_items_in_cache_)
-                             : 0;
+  size_t num_to_remove =
+      (decoded_images_.size() > limit) ? (decoded_images_.size() - limit) : 0;
   for (auto it = decoded_images_.rbegin();
        num_to_remove != 0 && it != decoded_images_.rend();) {
     if (it->second->is_locked()) {
@@ -815,6 +813,11 @@ void SoftwareImageDecodeCache::ReduceCacheUsage() {
   }
 }
 
+void SoftwareImageDecodeCache::ReduceCacheUsage() {
+  base::AutoLock lock(lock_);
+  ReduceCacheUsageUntilWithinLimit(max_items_in_cache_);
+}
+
 void SoftwareImageDecodeCache::RemovePendingTask(const ImageKey& key,
                                                  DecodeTaskType task_type) {
   base::AutoLock lock(lock_);
@@ -1173,7 +1176,11 @@ void SoftwareImageDecodeCache::OnMemoryStateChange(base::MemoryState state) {
         return;
     }
   }
-  ReduceCacheUsage();
+}
+
+void SoftwareImageDecodeCache::OnPurgeMemory() {
+  base::AutoLock lock(lock_);
+  ReduceCacheUsageUntilWithinLimit(0);
 }
 
 }  // namespace cc