Add more comments to content_capture_data.h

Change-Id: I5d5a979cb8360341c910288141394e84ebf4e2b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616389Reviewed-by: Changwan Ryu <changwan@chromium.org>
Reviewed-by: Tao Bai <michaelbai@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663931}

components/content_capture/common/content_capture_data.h

@@ -15,15 +15,20 @@ namespace content_capture {
 // This struct defines the on-screen text content and its bounds in a frame,
 // the text is captured in renderer and sent to browser; the root of
 // this tree is frame, the child could be the scrollable area or the text
-// content.
+// content, scrollable area can have scrollable area or text as child. Text
+// cannot have any child.
 struct ContentCaptureData {
   ContentCaptureData();
   ContentCaptureData(const ContentCaptureData& data);
   ~ContentCaptureData();
 
-  // The id of the frame or the content.
+  // The id of the frame or the content,
+  // For frame, this will be 0 until ContentCaptureReceiver assigns a unique ID.
   int64_t id = 0;
   // The url of frame or the text of the content.
+  // For frame, this is the URL of the frame.
+  // For scrollable area, this is not used.
+  // For text, this is the text value.
   base::string16 value;
   // The bounds of the frame or the content.
   gfx::Rect bounds;
@@ -38,6 +43,9 @@ struct ContentCaptureData {
 };
 
 // This defines a session, is a list of frames from current frame to root.
+// This represents the frame hierarchy, starting from the current frame to the
+// root frame, in the upward order. Note that ContentCaptureData here can only
+// have URL as value, and no ContentCaptureData has children in it.
 using ContentCaptureSession = std::vector<ContentCaptureData>;
 
 }  // namespace content_capture