Update comment on pdf_is_standalone in pdfium.gni

Change-Id: I40999031f7c63806af30952ff5719e0fb9ecdced
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/110093
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

pdfium.gni

diff --git a/pdfium.gni b/pdfium.gni
index 9c6ae8d..2296f2d 100644
--- a/pdfium.gni
+++ b/pdfium.gni
@@ -49,7 +49,8 @@
   # renderer is selectable at runtime.
   pdf_use_skia = pdf_use_skia_override
 
-  # Build PDFium standalone
+  # Build PDFium standalone. Now only controls whether the test binaries
+  # are built. Most logic is conditioned by build_with_chromium.
   pdf_is_standalone = false
 
   # Build a complete static library