Move CPWL_AppStream to CPDFSDK_AppStream.

No other fpdfsdk/pwl files use cpwl_appstream.h, but cpwl_appstream.cpp
contains many circular includes to the top-level fpdfsdk/ layer.
Since CPWL_AppStream is only used by this top-level layer, putting
it here makes dependencies saner.

Change-Id: I3cd6f8f823fd9e316e885f69a2afe51283f2f88e
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/58612
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/BUILD.gn b/fpdfsdk/BUILD.gn
index a8c7ec4..62df18b 100644
--- a/fpdfsdk/BUILD.gn
+++ b/fpdfsdk/BUILD.gn
@@ -22,6 +22,8 @@
     "cpdfsdk_annotiteration.h",
     "cpdfsdk_annotiterator.cpp",
     "cpdfsdk_annotiterator.h",
+    "cpdfsdk_appstream.cpp",
+    "cpdfsdk_appstream.h",
     "cpdfsdk_baannot.cpp",
     "cpdfsdk_baannot.h",
     "cpdfsdk_baannothandler.cpp",