ui/base/clipboard: Class comments describing Clipboard and subclasses.

Comment-only change.

Change-Id: Ia879b1b02f6d2d1dbcd7019ec2fff431285acacc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716285
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683013}

ui/base/clipboard/clipboard.h

@@ -34,6 +34,11 @@ namespace ui {
 class TestClipboard;
 class ScopedClipboardWriter;
 
+// Clipboard:
+// - reads from and writes to the system clipboard.
+// - specifies an ordering in which to write types to the clipboard
+//   (see |ObjectType|).
+// - is generalized for all targets/operating systems.
 class COMPONENT_EXPORT(BASE_CLIPBOARD) Clipboard : public base::ThreadChecker {
  public:
   static bool IsSupportedClipboardType(ClipboardType type) {

ui/base/clipboard/clipboard_format_type_aura.cc

@@ -12,11 +12,10 @@ namespace {
 constexpr char kMimeTypeFilename[] = "chromium/filename";
 }
 
-// I would love for the ClipboardFormatType to really be a wrapper around an X11
-// ::Atom, but there are a few problems. Chromeos unit tests spawn a new X11
-// server for each test, so Atom numeric values don't persist across tests. We
-// could still maybe deal with that if we didn't have static accessor methods
-// everywhere.
+// TODO(huangdarwin): Investigate ClipboardFormatType becoming a wrapper
+// around an X11 ::Atom. This wasn't possible in the past, because unit tests
+// spawned a new X11 server for each test, meaning Atom numeric values didn't
+// persist across tests.
 ClipboardFormatType::ClipboardFormatType() = default;
 
 ClipboardFormatType::~ClipboardFormatType() = default;

ui/base/clipboard/clipboard_ozone.h

@@ -14,6 +14,8 @@
 
 namespace ui {
 
+// ClipboardOzone is not yet shipped in production. It is a work in progress
+// for desktop Linux Wayland support.
 class ClipboardOzone : public Clipboard {
  private:
   friend class Clipboard;