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/gm/dashing.cpp b/gm/dashing.cpp
index 68fe7f1..a46d31f 100644
--- a/gm/dashing.cpp
+++ b/gm/dashing.cpp
@@ -512,7 +512,7 @@
     p.setStrokeWidth(1);
     const SkScalar intervals[] = { 1, 1 };
     p.setPathEffect(SkDashPathEffect::Make(intervals, SK_ARRAY_COUNT(intervals), 0));
-    
+
     canvas->translate(50, 50);
     canvas->drawPath(lines, p);
 }