Fix free memory calculation.

base/process/process_metrics.h

Index: base/process/process_metrics.h
diff --git a/base/process/process_metrics.h b/base/process/process_metrics.h
index 2448a715ace9b5eade7f09524561ffcedb666cc3..8e137db2f840bcf00e45770286d55158a0f05b4c 100644
--- a/base/process/process_metrics.h
+++ b/base/process/process_metrics.h
@@ -264,8 +264,9 @@ BASE_EXPORT void SetFdLimit(unsigned int max_descriptors);
 // Total/free memory are available on all platforms that implement
 // GetSystemMemoryInfo(). Total/free swap memory are available on all platforms
 // except on Mac. Buffers/cached/active_anon/inactive_anon/active_file/
-// inactive_file/dirty/pswpin/pswpout/pgmajfault are available on
+// inactive_file/dirty/reclaimable/pswpin/pswpout/pgmajfault are available on
 // Linux/Android/Chrome OS. Shmem/slab/gem_objects/gem_size are Chrome OS only.
+// Speculative/file_backed/purgeable are Mac and iOS only.
 struct BASE_EXPORT SystemMemoryInfoKB {
   SystemMemoryInfoKB();
   SystemMemoryInfoKB(const SystemMemoryInfoKB& other);
@@ -274,13 +275,29 @@ struct BASE_EXPORT SystemMemoryInfoKB {
   std::unique_ptr<Value> ToValue() const;
 
   int total;
+  // Note: on Windows this field also includes "zero" memory.
+  // See "Memory Management" and "Memory Performance Information"
+  // articles at MSDN.
   int free;
 
-#if defined(OS_LINUX)
+#if defined(OS_WIN)
+  // "This is the amount of physical memory that can be immediately reused
+  // without having to write its contents to disk first. It is the sum of the
+  // size of the standby, free, and zero lists." (MSDN).
+  // Standby: not modified pages of physical ram (file-backed memory) that are
+  // not actively being used.
+  // Note: on Windows this field has a bit different meaning from the Linux one
+  // (see below).
+  int available;
+#endif
+
+#if defined(OS_LINUX) || defined(OS_ANDROID)
   // This provides an estimate of available memory as described here:
   // https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773
   // NOTE: this is ONLY valid in kernels 3.14 and up.  Its value will always
   // be 0 in earlier kernel versions.
+  // Note: on Linux "available" includes _all_ file-backed memory
+  // (active + inactive). It is different from the Windows one (see above).
   int available;
 #endif
 
@@ -297,6 +314,7 @@ struct BASE_EXPORT SystemMemoryInfoKB {
   int active_file;
   int inactive_file;
   int dirty;
+  int reclaimable;
 
   // vmstats data.
   unsigned long pswpin;
@@ -311,6 +329,12 @@ struct BASE_EXPORT SystemMemoryInfoKB {
   int gem_objects;
   long long gem_size;
 #endif  // defined(OS_CHROMEOS)
+
+#if defined(OS_MACOSX) || defined(OS_IOS)
+  int speculative;
+  int file_backed;
+  int purgeable;
+#endif
 };
 
 // On Linux/Android/Chrome OS, system-wide memory consumption data is parsed