Re-land: base: Replace PurgeAndTruncate with Shrink function.

content/common/host_discardable_shared_memory_manager.cc

Index: content/common/host_discardable_shared_memory_manager.cc
diff --git a/content/common/host_discardable_shared_memory_manager.cc b/content/common/host_discardable_shared_memory_manager.cc
index c4fea8de54e0c7d3b2e2ab09e8485a2103a39756..1e5e75a5b7bc8194b873d25c0d636d564827f601 100644
--- a/content/common/host_discardable_shared_memory_manager.cc
+++ b/content/common/host_discardable_shared_memory_manager.cc
@@ -359,12 +359,8 @@ void HostDiscardableSharedMemoryManager::ReduceMemoryUsageUntilWithinLimit(
     scoped_refptr<MemorySegment> segment = segments_.back();
     segments_.pop_back();
 
-    // Attempt to purge and truncate LRU segment. When successful, as much
-    // memory as possible will be released to the OS. How much memory is
-    // released depends on the platform. The child process should perform
-    // periodic cleanup to ensure that all memory is release within a
-    // reasonable amount of time.
-    if (segment->memory()->PurgeAndTruncate(current_time)) {
+    // Attempt to purge LRU segment. When successful, released the memory.
+    if (segment->memory()->Purge(current_time)) {
       ReleaseMemory(segment->memory());
       continue;
     }
@@ -387,6 +383,11 @@ void HostDiscardableSharedMemoryManager::ReleaseMemory(
   DCHECK_GE(bytes_allocated_, size);
   bytes_allocated_ -= size;
 
+#if defined(OS_POSIX) && !defined(OS_ANDROID)

[danakj, 2015/03/19 16:41:56]

Hmm.. the only thing I like a bit less is these guards in content/ it would be
easy to let them get out of sync with the ones in base/. I would slightly prefer
that only the test can get out of sync and that it would fail when doing so.

[reveman, 2015/03/19 19:23:05]

Added DISCARDABLE_SHARED_MEMORY_SHRINKING define to prevent this from getting
out of sync. I could hide the ifdef inside the Shrink function and have Shrink
be a noop when not supported but I think I prefer to keep this explicit. This
code should be aware of the fact that shrinking is not always supported and we
might handle the case when it's not a bit differently in the future.

+  // Shrink memory segment. This will immediately release the memory to the OS.
+  memory->Shrink();
+#endif
+
   // This will unmap the memory segment and drop our reference. The result
   // is that the memory will be released to the OS if the child process is
   // no longer referencing it.