Use unique_ptrs for codec contexts in ccodec_progressivedecoder.

Change-Id: I4deaf8ca946c2dcb6842c5702b02eed1c22b1201
Reviewed-on: https://pdfium-review.googlesource.com/6191
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 9ecd20d..d54df57 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -86,6 +86,7 @@
   if (is_win) {
     cflags += [
       "/wd4267",
+      "/wd4324",
       "/wd4577",
     ]
   }