[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}

mojo/core/scoped_process_handle.cc

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