Add ifdef for jpeglibmangler

Bug: b/184745328

Only include jpeglibmangler when a new flag (MANGLE_JPEG_NAMES) is
defined. Define the flag in BUILD.gn. This will allow the Android
platform to sync to this version, not define the flag in its own build
files, and properly exclude jpeglibmangler.h.

Change-Id: I510b4abde046172f2b96c09a23481404402d2217
diff --git a/jpeglib.h b/jpeglib.h
index 979ef7c..a717928 100644
--- a/jpeglib.h
+++ b/jpeglib.h
@@ -19,7 +19,9 @@
 #define JPEGLIB_H
 
 /* Begin chromium edits */
+#ifdef MANGLE_JPEG_NAMES
 #include "jpeglibmangler.h"
+#endif
 /* End chromium edits */
 
 /*