Provide ascii-equivalents for FWL_VKEYCODE enums.

Help to distinguish more clearly between character codes and
virtual key codes. Then use them consistently. Then fix a few
places where the use was inconsistent.

Change-Id: I96f60e3931d4b1810d35c9a90708c74bc35daf97
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/83912
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/constants/BUILD.gn b/constants/BUILD.gn
index 1c7cea4..d6cd04a 100644
--- a/constants/BUILD.gn
+++ b/constants/BUILD.gn
@@ -12,6 +12,7 @@
     "annotation_flags.h",
     "appearance.cpp",
     "appearance.h",
+    "ascii.h",
     "form_fields.cpp",
     "form_fields.h",
     "form_flags.h",