Make PDF_USE_SKIA a public build definition

Move the build definition for PDF_USE_SKIA into the public
configuration. This is necessary to enable embedders to use the
public APIs which are controlled by it, such as FPDF_FFLDrawSkia()
and FPDF_RenderPageSkia().

Bug: pdfium:11
Change-Id: Ibf35432691fccecd8b4737e1a4737dd0fa9ae4ad
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/115870
Commit-Queue: Alan Screen <awscreen@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 2b1f97f..b4815b7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -40,12 +40,8 @@
     defines += [ "PDF_ENABLE_CLICK_LOGGING" ]
   }
 
-  if (pdf_use_skia) {
-    defines += [ "PDF_USE_SKIA" ]
-
-    if (pdf_enable_fontations) {
-      defines += [ "PDF_ENABLE_FONTATIONS" ]
-    }
+  if (pdf_use_skia && pdf_enable_fontations) {
+    defines += [ "PDF_ENABLE_FONTATIONS" ]
   }
 
   if (pdf_use_partition_alloc) {
@@ -261,6 +257,10 @@
       }
     }
   }
+
+  if (pdf_use_skia) {
+    defines += [ "PDF_USE_SKIA" ]
+  }
 }
 
 config("pdfium_core_config") {