Give the main frame a RenderWidget.

Currently, RenderView is a subclass of RenderWidget, and the RenderWidget
portion is effectively treated as the  the widget for the main frame as
well. For a number of reasons, this is a problematic model:

- A remote frame doesn't need a widget; however, a RenderView with a remote
  main frame still has a vestigal RenderWidget.
- Code that needs to affect both RenderWidget / RenderView is awkwardly
  split between them, in both content and blink.
- RenderView itself is often seen as an easy entry point to perform
  page-level work in the renderer. With OOPI, this is no longer a valid
  assumption.

In order to incrementally de-widgetize RenderView, the main frame will
have also have a RenderWidget, to make it consistent with the local frame
roots for subframes, which already have a RenderWidget. However, instead of
giving main frames their own RenderWidget, the main frame re-uses the
RenderView as its RenderWidget.

The rationale for taking this approach is to minimize the breakage: today,
Chrome simply doesn't expect to have two "widgets" for a frame.
Instantiating a distinct RenderWidget for the main frame can confuse code
that iterates or counts the active widgets: an example of this is the
security check for injecting WebUI bindings.

In the future, when RenderViewHost has-a RenderWidgetHost (and similarly,
when RenderViewImpl has-a RenderWidget) instead of today's is-a relation,
then it be conceptually much more straightforward to transition completely
to the new model.

BUG=419087

Review URL: https://codereview.chromium.org/1303773002

Cr-Commit-Position: refs/heads/master@{#356176}

1 files changed, 3 insertions, 2 deletions

diff --git a/content/public/browser/web_contents_delegate.h b/content/public/browser/web_contents_delegate.h
index eb5f480..40a4904 100644
--- a/content/public/browser/web_contents_delegate.h
+++ b/content/public/browser/web_contents_delegate.h
@@ -289,8 +289,9 @@ class CONTENT_EXPORT WebContentsDelegate {
   // be destroyed.
   virtual bool ShouldCreateWebContents(
       WebContents* web_contents,
-      int route_id,
-      int main_frame_route_id,
+      int32_t route_id,
+      int32_t main_frame_route_id,
+      int32_t main_frame_widget_route_id,
       WindowContainerType window_container_type,
       const std::string& frame_name,
       const GURL& target_url,