Replace defined(OS_APPLE) with BUILDFLAG(IS_APPLE).

Bug: chromium:1234043
Change-Id: I35f293235e7fad0547ca2aa8d377f9f2be9c43c6
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/89230
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxge/text_char_pos.h b/core/fxge/text_char_pos.h
index b28d249..6be2671 100644
--- a/core/fxge/text_char_pos.h
+++ b/core/fxge/text_char_pos.h
@@ -20,7 +20,7 @@
   uint32_t m_Unicode = 0;
   uint32_t m_GlyphIndex = 0;
   int m_FontCharWidth = 0;
-#if defined(OS_APPLE)
+#if BUILDFLAG(IS_APPLE)
   uint32_t m_ExtGID = 0;
 #endif
   int32_t m_FallbackFontPosition = 0;