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/src/shaders/gradients/SkRadialGradient.h b/src/shaders/gradients/SkRadialGradient.h
index 6a6dadf..1f6441b 100644
--- a/src/shaders/gradients/SkRadialGradient.h
+++ b/src/shaders/gradients/SkRadialGradient.h
@@ -26,7 +26,7 @@
     SkRadialGradient(SkReadBuffer& buffer);
     void flatten(SkWriteBuffer& buffer) const override;
     sk_sp<SkShader> onMakeColorSpace(SkColorSpaceXformer* xformer) const override;
-    
+
     void appendGradientStages(SkArenaAlloc* alloc, SkRasterPipeline* tPipeline,
                               SkRasterPipeline* postPipeline) const override;