Remove incorrect check in MHTMLBodyLoaderClient::Detach.

Bug: 1131879
Change-Id: I12e46e7c7edcbf41a6c51b5f2210a3eaed1412ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521872Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824580}

content/renderer/render_frame_impl.cc

@@ -1227,7 +1227,9 @@ class RenderFrameImpl::MHTMLBodyLoaderClient
   // Marks |this|'s pending load as abandoned. There are a number of reasons
   // this can happen; see the destructor for more information.
   void Detach() {
-    CHECK(frame_->in_frame_tree_);
+    // Note that the MHTMLBodyLoaderClient might be associated with a
+    // provisional frame, so this does not assert that `frame_->in_frame_tree_`
+    // is true.
     frame_ = nullptr;
   }