Enable gn check of //ppapi/proxy:proxy.

This CL trades the target-level allowlist for "nogncheck" on a few
lines. This allows us to turn on `gn check` for the target, and prevent
backsliding in other files, even while the leaky abstraction continues
to exist.

Bug: 1158995, 1228394
Change-Id: If147d215cdd0157175bbe9e9c9eba61dc7403547
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3018325
Auto-Submit: Chris Fredrickson <[email protected]>
Reviewed-by: Bill Budge <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Dirk Pranke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#900650}
diff --git a/ppapi/proxy/BUILD.gn b/ppapi/proxy/BUILD.gn
index 17211f375..7f4e104 100644
--- a/ppapi/proxy/BUILD.gn
+++ b/ppapi/proxy/BUILD.gn
@@ -16,8 +16,11 @@
     # easily make async callbacks in the proxy. We can't depend on the
     # full C++ wrappers at this layer since the C++ wrappers expect
     # symbols defining the globals for "being a plugin" which we are not.
-    # These callback files are standalone.
+    #
+    # These callback files are supposed to be standalone, but are not; see
+    # http://crbug.com/1228394.
     "../cpp/completion_callback.h",
+    "../cpp/logging.h",
     "../utility/completion_callback_factory.h",
     "audio_buffer_resource.cc",
     "audio_buffer_resource.h",