cc: Keep most recently used resources at the front of resource queue.

cc/resources/resource_pool.cc

Index: cc/resources/resource_pool.cc
diff --git a/cc/resources/resource_pool.cc b/cc/resources/resource_pool.cc
index 835c37d6c32d9f23029bcfea419ad6f8f2b3f2fc..2a9a10c7cc3f04c69293289b2d3961afdea695e9 100644
--- a/cc/resources/resource_pool.cc
+++ b/cc/resources/resource_pool.cc
@@ -91,9 +91,13 @@ ResourcePool::~ResourcePool() {
 
 Resource* ResourcePool::AcquireResource(const gfx::Size& size,
                                         ResourceFormat format) {
-  for (ResourceDeque::iterator it = unused_resources_.begin();
-       it != unused_resources_.end(); ++it) {
-    ScopedResource* resource = *it;
+  // Traverse the |unused_resources_| from rear end as MRU resources are
+  // held at the rear end of the queue. This touches LRU resources only if
+  // needed, which inreases possibility of expiring more LRU resources
+  // within kResourceExpirationDelayMs.
+  for (ResourceDeque::reverse_iterator rit = unused_resources_.rbegin();
+       rit != unused_resources_.rend(); ++rit) {
+    ScopedResource* resource = *rit;
     DCHECK(resource_provider_->CanLockForWrite(resource->id()));
 
     if (resource->format() != format)
@@ -101,6 +105,9 @@ Resource* ResourcePool::AcquireResource(const gfx::Size& size,
     if (resource->size() != size)
       continue;
 
+    // Get iterator from reverse_iterator pointing to the same resource.
+    ResourceDeque::iterator it =
+        static_cast<ResourceDeque::iterator>(--rit.base());

[reveman, 2015/11/02 06:07:51]

if we need this then can we add a base() function to ResourceDeque::iterator
instead of the static_cast?

[prashant.n, 2015/11/02 17:56:45]

IMO, iterators are different, so typecast would be good, to let user know what
is being done. Otherwise we need to modify scoped ptr deque.
WDYT?

     // Transfer resource to |in_use_resources_|.
     in_use_resources_.set(resource->id(), unused_resources_.take(it));
     in_use_memory_usage_bytes_ += ResourceUtil::UncheckedSizeInBytes<size_t>(