pdfium: Declare partition_alloc location.

partition_alloc library is being built as part of Chrome. It can
also be fetched independently inside pdfium.

This patch prepares Chrome to provide pdfium partition_alloc location.

Bug: None
Change-Id: Ia41eeab82cb85a2986ea8c20f46d1c89f3e72f3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5104710
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1235724}

.gn

@@ -66,7 +66,7 @@ default_args = {
       "//third_party/llvm-build/Release+Asserts/bin/llvm-readelf"
 
   # Overwrite default args declared in the pdfium library
-  pdf_use_partition_alloc_new_location = true
+  pdf_partition_alloc_dir = "//base/allocator/partition_allocator"
 
   devtools_visibility = [ "*" ]
 }