Remove trailing whitespace.

Also adds a presubmit to prevent adding trailing whitespace to source
code in the future.

Change-Id: I41a4df81487f6f00aa19b188f0cac6a3377efde6
Reviewed-on: https://skia-review.googlesource.com/57380
Reviewed-by: Ravi Mistry <rmistry@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/example/mac/HelloWorldNSView.mm b/example/mac/HelloWorldNSView.mm
index dcc82e3..63852e5 100644
--- a/example/mac/HelloWorldNSView.mm
+++ b/example/mac/HelloWorldNSView.mm
@@ -8,7 +8,7 @@
 #import "HelloWorldNSView.h"
 
 #include "SkApplication.h"
-#include <crt_externs.h>  
+#include <crt_externs.h>
 
 @implementation HelloWorldNSView