base: "implement" seek time detection for iOS/Android and add more stubs.

base/sys_info_mac.cc

Index: base/sys_info_mac.cc
diff --git a/base/sys_info_mac.cc b/base/sys_info_mac.cc
index 18df62482ea119db2d865f1745cf9de271767a19..86350880dc2fd6ccf9c9b7b7fe50ee7ca9fcb705 100644
--- a/base/sys_info_mac.cc
+++ b/base/sys_info_mac.cc
@@ -76,6 +76,11 @@ int64 SysInfo::AmountOfAvailablePhysicalMemory() {
       vm_info.free_count - vm_info.speculative_count) * PAGE_SIZE;
 }
 
+bool SysInfo::HasSeekPenalty(const FilePath& path, bool* has_seek_penalty) {
+  // TODO(dbeam): implement.
+  return false;
+}
+
 // static
 std::string SysInfo::CPUModelName() {
   char name[256];