Add comments for some blink classes: Page, LocalFrame, and RemoteFrame.

Change-Id: I8546a51ad0886f7c5f5acb0034f895d85a78a4d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366332
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799999}

third_party/blink/renderer/core/frame/local_frame.h

@@ -139,6 +139,7 @@ class WebURLLoaderFactory;
 
 extern template class CORE_EXTERN_TEMPLATE_EXPORT Supplement<LocalFrame>;
 
+// A LocalFrame is a frame hosted inside this process.
 class CORE_EXPORT LocalFrame final
     : public Frame,
       public FrameScheduler::Delegate,

third_party/blink/renderer/core/frame/remote_frame.h

@@ -32,6 +32,7 @@ class MessageEvent;
 class RemoteFrameClient;
 struct FrameLoadRequest;
 
+// A RemoteFrame is a frame that is possibly hosted outside this process.
 class CORE_EXPORT RemoteFrame final : public Frame,
                                       public mojom::blink::RemoteMainFrame,
                                       public mojom::blink::RemoteFrame {

third_party/blink/renderer/core/page/page.h

@@ -91,6 +91,10 @@ typedef uint64_t LinkHash;
 
 float DeviceScaleFactorDeprecated(LocalFrame*);
 
+// A Page roughly corresponds to a tab or popup window in a browser. It owns a
+// tree of frames (a blink::FrameTree). The root frame is called the main frame.
+//
+// Note that frames can be local or remote to this process.
 class CORE_EXPORT Page final : public GarbageCollected<Page>,
                                public Supplementable<Page>,
                                public SettingsDelegate,