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/gn/effects.gni b/gn/effects.gni
index f3a480d..138b5b5 100644
--- a/gn/effects.gni
+++ b/gn/effects.gni
@@ -46,7 +46,6 @@
   "$_src/effects/SkMorphologyImageFilter.cpp",
   "$_src/effects/SkOffsetImageFilter.cpp",
   "$_src/effects/SkOverdrawColorFilter.cpp",
-  "$_src/effects/SkOverdrawColorFilter.h",
   "$_src/effects/SkPackBits.cpp",
   "$_src/effects/SkPackBits.h",
   "$_src/effects/SkPaintFlagsDrawFilter.cpp",
@@ -106,6 +105,7 @@
   "$_include/effects/SkMagnifierImageFilter.h",
   "$_include/effects/SkMorphologyImageFilter.h",
   "$_include/effects/SkOffsetImageFilter.h",
+  "$_include/effects/SkOverdrawColorFilter.h",
   "$_include/effects/SkPaintFlagsDrawFilter.h",
   "$_include/effects/SkPaintImageFilter.h",
   "$_include/effects/SkPerlinNoiseShader.h",