content: update comment in UpdateBrowserControlsState

The issue is the LocalMainFrame interface is registered on the
renderer side once the LocalFrame is created. This means that if
LocalMainFrame is requested before the renderer is created, any
calls sent along the interface are effectively dropped. I tried adding
a DCHECK in GetAssociatedLocalMainFrame() that it's only called if
IsRenderFrameCreated() is true, but that hits in tests.

BUG=1154852
TEST=comment change only

Change-Id: Id602b1883c7b554f07fc50ecc78c7fa8e041602b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585306Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835814}

content/browser/renderer_host/render_frame_host_impl.cc

@@ -4499,8 +4499,8 @@ void RenderFrameHostImpl::UpdateBrowserControlsState(
     bool animate) {
   DCHECK(frame_tree_node_->IsMainFrame());
 
-  // TODO(https://crbug.com/1154852): Don't update browser control state for a
-  // frame that hasn't been created yet.
+  // TODO(https://crbug.com/1154852): Asking for the LocalMainFrame interface
+  // before the RenderFrame is created is racy.
   if (!IsRenderFrameCreated())
     return;