Clean up a few includes, introduce iwyu.

src/ports/SkOSFile_posix.cpp

Index: src/ports/SkOSFile_posix.cpp
diff --git a/src/ports/SkOSFile_posix.cpp b/src/ports/SkOSFile_posix.cpp
index cea688b86f4e331c46d0e014739abd9b33d21956..2dce913e4d20df4cb26543c3a0f3d80804377f90 100644
--- a/src/ports/SkOSFile_posix.cpp
+++ b/src/ports/SkOSFile_posix.cpp
@@ -7,16 +7,19 @@
 
 #include "SkOSFile.h"
 
-#include "SkTFitsIn.h"
-#include "SkTypes.h"
-
 #include <dirent.h>
 #include <stdio.h>
+#include <string.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
+#include "SkString.h"
+#include "SkTFitsIn.h"
+#include "SkTemplates.h"
+#include "SkTypes.h"
+
 bool sk_exists(const char *path, SkFILE_Flags flags) {
     int mode = F_OK;
     if (flags & kRead_SkFILE_Flag) {