Remove unnecessary includes in src/pdf/

Use forward declaration more.

BUG=None
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2110033002

Review-Url: https://codereview.chromium.org/2110033002
diff --git a/src/pdf/SkPDFGraphicState.h b/src/pdf/SkPDFGraphicState.h
index 06f1e11..569f9da 100644
--- a/src/pdf/SkPDFGraphicState.h
+++ b/src/pdf/SkPDFGraphicState.h
@@ -9,10 +9,10 @@
 #ifndef SkPDFGraphicState_DEFINED
 #define SkPDFGraphicState_DEFINED
 
-#include "SkPaint.h"
 #include "SkPDFStream.h"
 #include "SkChecksum.h"
 
+class SkPaint;
 class SkPDFCanon;
 class SkPDFFormXObject;