remove main_render_frame(), use GetMainRenderFrame() instead

CL 93333002 added a main_render_frame() method to RenderFrameImpl,
then about a week later CL 107893003 added a virtual
GetMainRenderFrame() method with the same implementation.
To avoid confusion, we could just use the latter and remove
main_render_frame().

BUG=304341

Review URL: https://codereview.chromium.org/554743009

Cr-Commit-Position: refs/heads/master@{#294562}
diff --git a/content/renderer/render_view_impl.cc b/content/renderer/render_view_impl.cc
index f0865b0d..59c5664f 100644
--- a/content/renderer/render_view_impl.cc
+++ b/content/renderer/render_view_impl.cc
@@ -1966,7 +1966,7 @@
   FOR_EACH_OBSERVER(RenderViewObserver, observers_, FocusedNodeChanged(node));
 
   // TODO(dmazzoni): this should be part of RenderFrameObserver.
-  main_render_frame()->FocusedNodeChanged(node);
+  GetMainRenderFrame()->FocusedNodeChanged(node);
 }
 
 void RenderViewImpl::didUpdateLayout() {
@@ -2463,7 +2463,7 @@
   return RenderWidget::Send(message);
 }
 
-RenderFrame* RenderViewImpl::GetMainRenderFrame() {
+RenderFrameImpl* RenderViewImpl::GetMainRenderFrame() {
   return main_render_frame_.get();
 }
 
@@ -4038,7 +4038,7 @@
   // Never show a disambiguation popup when accessibility is enabled,
   // as this interferes with "touch exploration".
   AccessibilityMode accessibility_mode =
-      main_render_frame()->accessibility_mode();
+      GetMainRenderFrame()->accessibility_mode();
   bool matches_accessibility_mode_complete =
       (accessibility_mode & AccessibilityModeComplete) ==
           AccessibilityModeComplete;