Avoid -Wcharacter-conversion

Followup to https://pdfium-review.googlesource.com/c/pdfium/+/133330,
this uses the same strategy to avoid the warning because we're not
actually treating these as characters.

Bug: 417996267
Change-Id: I0eb698726a3390fca34644f86c4f59cc1c9ff10b
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/133630
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxcrt/code_point_view.cpp b/core/fxcrt/code_point_view.cpp
index 417e1d0..12b3783 100644
--- a/core/fxcrt/code_point_view.cpp
+++ b/core/fxcrt/code_point_view.cpp
@@ -25,7 +25,7 @@
   if (current_.IsEmpty()) {
     return kSentinel;
   }
-  char32_t code_point = current_.Front();
+  uint32_t code_point = current_.Front();
   next_ = current_.Substr(1);
   if (IsHighSurrogate(code_point)) {
     if (!next_.IsEmpty() && IsLowSurrogate(next_.Front())) {