Make PDFium's compiler_specific.h use clang's UNSAFE_BUFFERS_BUILD

This symbol is defined consistently in build/config/clang/BUILD.gn
and is a better source of truth than the old PDFium symbol.

This allows the UNSAFE_BUFFERS macro to be correctly defined even
outside of pdfium_common_config, such as for fuzzers and testing.

Change-Id: I5f15147e8eeb28104323b5a58f09707a8df6de7e
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/127730
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index c962350..55422d6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -226,8 +226,6 @@
   }
 
   if (clang_use_chrome_plugins) {
-    defines += [ "PDF_USE_CHROME_PLUGINS" ]
-
     # Catch misuse of C-style pointers.
     # TODO(crbug.com/1320670): enable for non-debug builds once this stops
     # interfering with code generation.

core/fxcrt/compiler_specific.h

diff --git a/core/fxcrt/compiler_specific.h b/core/fxcrt/compiler_specific.h
index eb00fc4..5e2b033 100644
--- a/core/fxcrt/compiler_specific.h
+++ b/core/fxcrt/compiler_specific.h
@@ -86,7 +86,7 @@
 // clang-format off
 // Formatting is off so that we can put each _Pragma on its own line, as
 // recommended by the gcc docs.
-#if defined(PDF_USE_CHROME_PLUGINS)
+#if defined(UNSAFE_BUFFERS_BUILD)
 #define UNSAFE_BUFFERS(...)                  \
   _Pragma("clang unsafe_buffer_usage begin") \
   __VA_ARGS__                                \