Add comments for renderview vs renderwidget.

Change-Id: I64fc3fb04dd2e3e3f827f2c0a72917471ac0a403
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761329Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688361}

content/renderer/render_view_impl.h

@@ -92,6 +92,10 @@ class CreateViewParams;
 // a local frame for this view, then it also manages a RenderWidget for the
 // main frame.
 //
+// The main distinction between RenderView and RenderWidget is that the
+// RenderView holds synchronized state across all processes participating in the
+// frame tree, whereas the RenderWidget holds per-root-frame state.
+//
 // TODO(419087): Currently even though the RenderViewImpl "manages" the
 // RenderWidget, the RenderWidget owns the RenderViewImpl. This is due to
 // RenderViewImpl historically being a subclass of RenderWidget. Breaking