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/hardstop_gradients.cpp b/gm/hardstop_gradients.cpp
index 2ab2ed9..84b7ab8 100644
--- a/gm/hardstop_gradients.cpp
+++ b/gm/hardstop_gradients.cpp
@@ -58,7 +58,7 @@
 static void shade_rect(SkCanvas* canvas, sk_sp<SkShader> shader, int cellRow, int cellCol) {
     SkPaint paint;
     paint.setShader(shader);
-    
+
     SkRect rect = SkRect::MakeXYWH(SkIntToScalar(cellCol * CELL_WIDTH  + PAD_WIDTH),
                                    SkIntToScalar(cellRow * CELL_HEIGHT + PAD_HEIGHT),
                                    SkIntToScalar(RECT_WIDTH),
@@ -154,7 +154,7 @@
 
                 shade_rect(canvas, shader, cellRow, cellCol);
             }
-        } 
+        }
     }
 
 private: