AW NS: document StreamReaderUrlLoader path

No change to logic, only docs.

This updates the class doc for AndroidStreamReaderURLLoader, since the
doc was written back when it only supported shouldInterceptRequest. This
adds documentation for the known subclasses of ResponseDelegate, to
clarify which production cases they actually cover.

Test: None
Change-Id: Ib04e7dc6a43b548856d5bc2d41d9addabe824029
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717440Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681059}

android_webview/browser/network_service/android_stream_reader_url_loader.h

@@ -17,9 +17,15 @@ namespace android_webview {
 class InputStream;
 class InputStreamReaderWrapper;
 
-// Custom URLLoader implementation for loading network responses from stream.
-// Current implementation is in particular for supporting shouldInterceptRequest
-// callback in webview.
+// Custom URLLoader implementation for loading responses from Android
+// InputStreams. Although this works generally for implementers of the
+// ResponseDelegate interface, this specifically aims to support:
+//
+//  - shouldInterceptRequest callback
+//  - content:// URLs, which load content from Android ContentProviders (which
+//    could be in-app or come from other apps)
+//  - file:///android_asset/ & file:///android_res/ URLs, which load in-app
+//    content from the app's asset/ and res/ folders
 class AndroidStreamReaderURLLoader : public network::mojom::URLLoader {
  public:
   // Delegate abstraction for obtaining input streams.

android_webview/browser/network_service/aw_proxying_url_loader_factory.cc

@@ -150,6 +150,7 @@ class InterceptedRequest : public network::mojom::URLLoader,
   DISALLOW_COPY_AND_ASSIGN(InterceptedRequest);
 };
 
+// A ResponseDelegate for responses returned by shouldInterceptRequest.
 class InterceptResponseDelegate
     : public AndroidStreamReaderURLLoader::ResponseDelegate {
  public:
@@ -203,6 +204,9 @@ class InterceptResponseDelegate
   base::WeakPtr<InterceptedRequest> request_;
 };
 
+// A ResponseDelegate based on top of AndroidProtocolHandler for special
+// protocols, such as content://, file:///android_asset, and file:///android_res
+// URLs.
 class ProtocolResponseDelegate
     : public AndroidStreamReaderURLLoader::ResponseDelegate {
  public: