base: Add ashmem support to base::DiscardableSharedMemory implementation.

base/process/process_metrics_win.cc

Index: base/process/process_metrics_win.cc
diff --git a/base/process/process_metrics_win.cc b/base/process/process_metrics_win.cc
index 16db44f63f6e94b51034817a3e1eb1c72c72eb68..8e1d250d481d0724cba9973ea31fca2a442b20c4 100644
--- a/base/process/process_metrics_win.cc
+++ b/base/process/process_metrics_win.cc
@@ -284,4 +284,10 @@ size_t GetSystemCommitCharge() {
   return (info.CommitTotal * system_info.dwPageSize) / 1024;
 }
 
+size_t GetPageSize() {
+  SYSTEM_INFO system_info = {};
+  GetSystemInfo(&system_info);

[danakj, 2014/12/17 21:40:17]

http://msdn.microsoft.com/en-us/library/windows/desktop/ms724381%28v=vs.85%29...
suggests using GetNativeSystemInfo to get something more accurate, can you leave
a comment explaining why this one is correct?

[reveman, 2014/12/17 22:20:30]

I simply changed this to use PAGESIZE_KB in latest patch. I assume that is
always correct on win. If not, then a lot of other code in this file is also
broken.

+  return system_info.dwPageSize ? system_info.dwPageSize : 4096;

[danakj, 2014/12/17 21:40:17]

I can't see anything that suggests the return value for this can be 0. Can you
add a comment explaining when it would be 0, and why 4096 is correct in that
case?

[reveman, 2014/12/17 22:20:30]

Apparently GetSystemInfo is hookable/patchable and a return value of 0 would be
really bad. Good call though. This would have needed a comment at minimum if we
were to land it.

+}
+
 }  // namespace base