Revert "[mojo-core] PCHECK for invalid process handles"

This reverts commit c2c962cc.

Reason for revert: The crash being debugged was fixed by https://chromium-review.googlesource.com/c/1310505

Original change's description:
> [mojo-core] PCHECK for invalid process handles
>
> Changes a DCHECK to a PCHECK in an attempt to help sort out some crashes
> we see in the wild where the broker process apparently receives handles
> but doesn't attempt to duplicate them to itself. This should only be
> possible if it doesn't know a valid handle for the remote (sending)
> process, which points to potential application raciness in ownership of
> the process handles passed into Mojo.
>
> Bug: 887576
> Change-Id: I1f8786af8170b7db03804a98125365eeb44fc598
> Reviewed-on: https://chromium-review.googlesource.com/1241197
> Commit-Queue: Ken Rockot <rockot@chromium.org>
> Reviewed-by: Reilly Grant <reillyg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#593733}

R=rockot@google.com,reillyg@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 887576
Change-Id: Ic31379f9a85a20c8c47caccc185180768156eaa4
Reviewed-on: https://chromium-review.googlesource.com/c/1318069Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605351}

mojo/core/scoped_process_handle.cc

@@ -48,10 +48,7 @@ ScopedProcessHandle ScopedProcessHandle::CloneFrom(base::ProcessHandle handle) {
   BOOL ok = ::DuplicateHandle(GetCurrentProcessHandle(), handle,
                               GetCurrentProcessHandle(), &handle, 0, FALSE,
                               DUPLICATE_SAME_ACCESS);
-
-  // TODO(https://crbug.com/887576): Revert this to a DCHECK once we have sorted
-  // out the cause of handle verifier failures.
-  PCHECK(ok);
+  DCHECK(ok);
 #endif
   return ScopedProcessHandle(handle);
 }