Convert DCHECK to CHECK to validate cause of HttpCacheTransaction bug

DoneHeadersAddToEntryComplete assumes that the previous result is OK.
However, it may be ERR_CACHE_RACE in some situations, resulting in a
crash. Converting to a CHECK would verify that this is the cause of
some crashes observed in the wild.

Bug: 959194
Change-Id: Ib989e85b5334667875e8669f442f32c274b0d23c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773163Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690762}

net/http/http_cache_transaction.cc

@@ -1427,7 +1427,7 @@ int HttpCache::Transaction::DoDoneHeadersAddToEntryComplete(int result) {
   // created a new ActiveEntry (new_entry_) to write to (and doomed the old
   // one). Now that the new entry has been created, start writing the response.
 
-  DCHECK_EQ(result, OK);
+  CHECK_EQ(result, OK);
   DCHECK_EQ(mode_, WRITE);
   DCHECK(new_entry_);
   DCHECK(response_.headers);