Revert "Remove CHECK for same-document provisional frames for M87 beta."

This reverts commit 47c81eeb.

Reason for revert: Was landed for M87 branch

Original change's description:
> Remove CHECK for same-document provisional frames for M87 beta.
>
> R=​nasko@chromium.org
>
> Bug: 1125106
> Change-Id: If05f064715add137e16118a08da153d1d5ed0e3e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2465809
> Reviewed-by: Nasko Oskov <nasko@chromium.org>
> Commit-Queue: danakj <danakj@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#816752}

TBR=danakj@chromium.org,nasko@chromium.org

Change-Id: I6a37a0026dde48dd06f07d26aaed140c8713c36e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1125106
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468243Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816766}

content/renderer/render_frame_impl.cc

@@ -3649,9 +3649,7 @@ void RenderFrameImpl::CommitSameDocumentNavigation(
   DCHECK(!commit_params->is_view_source);
   DCHECK(NavigationTypeUtils::IsSameDocument(common_params->navigation_type));
 
-  // TODO(danakj): Disabled for M87, but underlying problem exists.
-  // CHECK(in_frame_tree_);
-
+  CHECK(in_frame_tree_);
   // Unlike a cross-document navigation commit, detach the MHTMLBodyLoaderClient
   // before resetting it. In the case of a cross-document navigation, it's
   // important to ensure *something* commits, even if the original commit