Fix free memory calculation.

base/sys_info.h

Index: base/sys_info.h
diff --git a/base/sys_info.h b/base/sys_info.h
index e35feff735e4ac3783d1870ae04d711ef7cdd7fc..c066abeef7f35c68a05a4f8bcab6d78c92342077 100644
--- a/base/sys_info.h
+++ b/base/sys_info.h
@@ -13,11 +13,20 @@
 
 #include "base/base_export.h"
 #include "base/files/file_path.h"
+#include "base/gtest_prod_util.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 
+FORWARD_DECLARE_TEST(SysInfoTest, AmountOfAvailablePhysicalMemory);
+
 namespace base {
 
+namespace debug {
+FORWARD_DECLARE_TEST(SystemMetricsTest, ParseMeminfo);
+}
+
+struct SystemMemoryInfoKB;
+
 class BASE_EXPORT SysInfo {
  public:
   // Return the number of logical processors/cores on the current machine.
@@ -28,6 +37,9 @@ class BASE_EXPORT SysInfo {
 
   // Return the number of bytes of current available physical memory on the
   // machine.
+  // (The amount of memory that can be allocated without any significant
+  // impact on the system. It can lead to freeing inactive file-backed
+  // and/or speculative file-backed memory).
   static int64_t AmountOfAvailablePhysicalMemory();
 
   // Return the number of bytes of virtual memory of this process. A return
@@ -147,6 +159,15 @@ class BASE_EXPORT SysInfo {
   // Low-end device refers to devices having less than 512M memory in the
   // current implementation.
   static bool IsLowEndDevice();
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(::SysInfoTest, AmountOfAvailablePhysicalMemory);
+  FRIEND_TEST_ALL_PREFIXES(::base::debug::SystemMetricsTest, ParseMeminfo);
+
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+  static int64_t AmountOfAvailablePhysicalMemory(
+      const SystemMemoryInfoKB& meminfo);
+#endif
 };
 
 }  // namespace base