Use safe bool conversion operators in fpdf_page/

core/fpdfapi/fpdf_page/cpdf_colorstate.h

Index: core/fpdfapi/fpdf_page/cpdf_colorstate.h
diff --git a/core/fpdfapi/fpdf_page/cpdf_colorstate.h b/core/fpdfapi/fpdf_page/cpdf_colorstate.h
index d928dbdb2ea47724a3788fd4526f72c81148e2ac..a4077d965f7f3c62e6822d09f14e25ad9094be04 100644
--- a/core/fpdfapi/fpdf_page/cpdf_colorstate.h
+++ b/core/fpdfapi/fpdf_page/cpdf_colorstate.h
@@ -51,7 +51,7 @@ class CPDF_ColorState {
   // TODO(tsepez): Stop leaking ColorStateData outside this class.
   const CPDF_ColorStateData* GetObject() const { return m_Ref.GetObject(); }
 
-  operator bool() const { return !!m_Ref; }
+  explicit operator bool() const { return !!m_Ref; }
 
  private:
   void SetColor(CPDF_Color& color,