Add Linux MemAvailable to SystemMemoryInfoKB

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 8d4e51b517d78ed4cb497b983cadb8325f35addc..57cb3abec0d20e3249d029da319c96f2e10ab58b 100644
--- a/base/process/process_metrics.h
+++ b/base/process/process_metrics.h
@@ -272,6 +272,14 @@ struct BASE_EXPORT SystemMemoryInfoKB {
   int total;
   int free;
 
+#if defined(OS_LINUX)
+  // 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.
+  int available;
+#endif
+
 #if !defined(OS_MACOSX)
   int swap_total;
   int swap_free;