Add SK_API to SkOverdrawCanvas and SkOverdrawColorFilter.

The SkiaRenderer in chromium is going to use the overdraw canvas and
filter for measuring overdraw. Move these headers out of src/ into
include/.

Bug: chromium:704285
Change-Id: I2abb1671b73e3d26552462cf700340a7e3b874f0
Reviewed-on: https://skia-review.googlesource.com/36160
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/ports/SkGlobalInitialization_default.cpp b/src/ports/SkGlobalInitialization_default.cpp
index d372f122..52ef365 100644
--- a/src/ports/SkGlobalInitialization_default.cpp
+++ b/src/ports/SkGlobalInitialization_default.cpp
@@ -33,7 +33,7 @@
 #include "SkMergeImageFilter.h"
 #include "SkMorphologyImageFilter.h"
 #include "SkOffsetImageFilter.h"
-#include "../../src/effects/SkOverdrawColorFilter.h"
+#include "SkOverdrawColorFilter.h"
 #include "SkPaintImageFilter.h"
 #include "SkPerlinNoiseShader.h"
 #include "SkPictureImageFilter.h"